OBJECTIVE: We tested the hypothesis that altering the pre-exercise muscle temperature would influence the magnitude of muscle damage induced by eccentric exercise. SUBJECTS: Female students who had no experience in resistance training were placed into either a microwave treatment group (n = 10) or an icing treatment group (n = 10). DESIGN AND SETTING: Subjects in each group performed 12 maximal eccentric actions of the forearm flexors of each arm on 2 separate occasions separated by 4 weeks. Before testing, the exercise arm was subjected to either passive warming (microwave) or control for the microwave treatment group or cooling (icing) or control for the icing treatment group. The control arm performed the same exercise protocol without treatment. Limbs were randomized for treatment or control and order of testing. Deep muscle temperature increased by approximately 3 degrees C after the microwave treatment and decreased approximately 5 degrees C after the icing treatment. MEASUREMENTS: We evaluated changes in maximal isometric force and indirect markers of muscle damage, including range of motion, upper arm circumference, muscle soreness, and plasma creatine kinase activity, in the microwave and control and icing and control groups using a 2-way, repeated-measures analysis of variance. RESULTS: All measures changed significantly (P <.01) after exercise, but neither of the treatments demonstrated significant effects on most of the variables compared with the control. CONCLUSIONS: These results suggest that pre-exercise cooling does not affect the magnitude of muscle damage in response to eccentric exercise. Similarly, pre-exercise passive muscle warming did not prove beneficial in attenuating indicators of muscle damage. Thus, any beneficial effects of warm-up exercise cannot be attributed to the effects of increased muscle temperature.