OPINION
MAUZY, Justice.This cause involves a City’s authority to adopt its own procedure to determine whether fire fighters are fit to continue their duties. Lester W. Tyra and others (Tyra), members of the City of Houston Fire Department, filed suit against the City of Houston and others (the City) to enjoin the fire chief’s implementation of certain test procedures. The court of appeals reversed the trial court judgment which granted the requested relief. 786 S.W.2d 457. We reverse the judgment of the court of appeals and remand this cause to the trial court.
In 1987, the chief of the Houston Fire Department issued Department Order No. 4, requiring all personnel involved in fire suppression to take annual tests. Tyra challenged the order, asserting that the tests would subject fire fighters to undue risk of physical injury, that blood pressure standards set out in the order were arbitrary and discriminatory, and that the order was an attempt to circumvent the Fire Fighters’ and Police Officers’ Civil Service Act, Tex.Loc.Gov’t Code ch. 143 (formerly Tex.Rev.Civ.Stat.Ann. art. 1269m (Vernon 1963)) (“the Act”). The trial court rendered judgment for the fire fighters and permanently enjoined the fire chief from implementing or applying Order No. 4.
While this case was pending before the court of appeals, the Texas legislature amended the Act by adding section 143.-1115, entitled “Determination of Physical *627and Mental Fitness.” Act of May 23, 1989, ch. 890, § 2,1989 Tex.Gen.Laws 3899.1 By its terms, section 143.1115 “provides the exclusive procedure for determining whether a fire fighter or police officer is sufficiently physically or mentally fit to continue the person’s duties or assignment.” § 143.1115(a).2 Section 143.1115 applies only to a municipality having a population of 1.5 million or more; i.e., the City of Houston. Tex.Loc.Gov’t Code Ann. § 143.-101. The court of appeals noted the adoption of section 143.1115, but decided that Order No. 4 was not inconsistent with the new, statutory procedure. 786 S.W.2d at 461.
Tyra argues that by enacting section 143.1115, the legislature has withdrawn the City’s authority to create its own procedures to determine whether a fire fighter is sufficiently physically or mentally fit to continue to perform his or her duties or assignment. We agree.
Initially, we note that section 143.1115 does not prohibit a police or fire department from adopting programs for the purposes of routine training or skills assessment. Such programs may provide valuable, systematic exercises, thus promoting the Act’s aim of “securpng] efficient fire and police departments composed of capable personnel.” Tex.Loc.Gov’t Code Ann. § 143.001(a).
Nor does section 143.1115 prohibit the suspension of a fire fighter or police officer who disobeys a rule or order requiring participation in such a program. The Act authorizes a local civil service commission to provide for removal or suspension of a fire fighter or police officer for “violation of an applicable fire or police department rule or special order.” Tex.Loc.Gov’t Code Ann. §§ 143.008(c), 143.051(12). The rule or order need only bear a reasonable relationship to the maintenance of an efficient fire fighting force. Jackson v. Firemen’s & Policemen’s Civil Service Commission of Galveston, 466 S.W.2d 412, 416 (Tex.1971). Thus, a fire fighter may be suspended for violation of a fire department rule or order even though the underlying grounds for suspension are not specifically enumerated in the Act. Jackson, 466 S.W.2d at 416; see also City of Sweetwater v. Geron, 380 S.W.2d 550 (Tex.1964).
Section 143.1115 does, however, prohibit a fire or police department from suspending or terminating an officer for failing to pass a physical or mental fitness test *628by some procedure other than that set forth in the Act. Such a test would obviously thwart the legislature’s express intent to establish “the exclusive procedure for determining whether a fire fighter or police officer is sufficiently physically or mentally fit to continue the person’s duties or assignment.” Téx.Loc.Gov’t Code Ann. § 143.1115(a).
Order No. 4 provides a detailed procedure for determining whether a fire fighter is sufficiently physically fit to continue the person’s duties or assignment. Section 6 of the order, entitled “Procedure,” requires a fire fighter to meet certain blood pressure requirements before taking the test. Section 6 further describes a series of physical tasks which the fire fighter must perform within a prescribed time limit. For each task, the order lists “Physical Abilities Measured,” such as leg strength, back strength, and cardiovascular endurance. The order also lists “Physical Abilities Required,” such as grip, arm, and chest strength.3 A fire fighter may fail the test twice without being suspended; but after the third failure, the officer administering the test is directed to “make a written recommendation to the Fire Chief for a one (1) day (12 hours) suspension.” (Emphasis in original.) The fourth and fifth failures result in recommendations of three-day and fifteen-day suspensions; and after the sixth failure, the administering officer “recommends to [the] Fire Chief that [the] firefighter be indefinitely suspended.” (Emphasis in original.)
The Texas Constitution prohibits a city from acting in a manner inconsistent with the general laws of the state. Tex. Const, art. XI, § 5. Thus, the legislature may, by general law, withdraw a particular subject from a home rule city’s domain. Glass v. Smith, 150 Tex. 632, 637, 244 S.W.2d 645, 649 (1951). When it does so, the legislature must express its intent in clear and unmistakable language. Id., 150 Tex. at 641, 244 S.W.2d at 652.
The language of section 143.1115 is both clear and unmistakable. By providing in that section the “exclusive procedure for determining whether a fire fighter or police officer is sufficiently physically or mentally fit to continue the person’s duties or assignment,” the legislature has withdrawn the City’s authority to create its own procedures for that purpose.
The City does, of course, retain the authority to implement the statutory procedure. In so doing, the City may still devise and require performance tests like Order No. 4, but without the sanction of suspension for inadequate performance. A fire fighter who performs unsatisfactorily on a test may not be summarily suspended in disregard of the Act; rather, the fire fighter must be afforded the procedural safeguards specified in section 143.1115.
We conclude that the City can require fire fighters to perform the tests set out in Order No. 4, but cannot suspend or sanction fire fighters without complying with the procedural mechanism outlined in section 143.1115.4 We reverse the judgment of the court of appeals and remand this cause to the trial court for further proceedings in accordance with this opinion.
. Although laws may not operate retroactively to impair vested substantive rights acquired under existing laws, changes in statutes affecting remedies or procedure may be applied retroactively. Holder v. Wood, 714 S.W.2d 318, 319 (Tex.1986); Ex Parte Abell, 613 S.W.2d 255, 260 (Tex.1981). In this case, section 143.1115 affects only the procedure by which Tyra’s physical capabilities are evaluated. Thus, none of Tyra's substantive rights are impinged, and the retroactive application of section 143.1115 does not affect our evaluation of this case.
. Specifically, the section prescribes the following steps:
(b) On receiving a written order by the department head, a fire fighter or police officer shall submit to the commission a report from the person’s personal physician, psychiatrist, or psychologist, as appropriate.
(c) If the commission, the department head, or the fire fighter or police officer questions the report, the commission shall appoint a physician, psychiatrist, or psychologist, as appropriate, to examine the fire fighter or police officer and to submit a report to the commission, the department head, and the person.
(d)If the report of the appointed physician, psychiatrist, or psychologist, as appropriate, disagrees with the report of the fire fighter’s or police officer’s personal physician, psychiatrist, or psychologist, as appropriate, the commission shall appoint an independent three-member board composed of a physician, a psychiatrist, and a psychologist or any combination, as appropriate, to examine the fire fighter or police officer. The board shall submit to the commission a written report of its finding regarding whether the fire fighter is sufficiently physically or mentally fit to continue the person’s duties or assignment. The commission, at its next regularly scheduled meeting after the date it receives the report of the board, shall determine whether the fire fighter or police officer is sufficiently physically or mentally fit to continue the person’s duties or assignment. The commission shall base its determination exclusively on the report of the board.
Section 143.1115 further provides for division of costs, subsec. (e), and prohibits appointment to the review board of any person receiving other compensation from the city, subsec. (f).
. The physical demands of Order No. 4 apparently colored the City’s perception of the fire fighters’ complaints. In oral argument before this Court, counsel for the City announced, "In our office we refer to this case as the case of the fat firemen.” Though apparently offered to belittle the fire fighter’s complaints, the City’s remark serves to confirm that Order No. 4 is indeed a test of physical fitness, and not a mere test of fire fighters’ skills. Ignoring that fact, the dissent relays the disparaging comment without discussing its relevance to the issue presented; indeed, the dissent does not even attribute the remark to its source.
. To avoid this result, the dissent takes a mystifying approach to legislative intent. The dissent acknowledges that section 143.1115 was adopted for the specific purpose of correcting problems in Order no. 4. Nonetheless, the dissent concludes that section 143.1115 does not even apply to Order no. 4, and criticizes the court for applying the statute in the very manner the legislature contemplated. As we pointed out recently in Lee v. City of Houston, 807 S.W.2d 290, 293 (Tex.1991), the function of this court is not to question the wisdom of a statute; rather, once we have determined the legislature’s intent, we must apply the statute as written. See Jones v. Del Ray Andersen and Associates, 539 S.W.2d 348, 351 (Tex.1976).