Differences in exercise-induced muscle damage between males and females were studied in an animal model by measuring the serum activities of specific muscle enzymes (creatine kinase and aspartate aminotransferase). It was found that rats showed muscle damage, comparable to that observed in humans after long-term exercise: males were much more affected than females. For example, the CK activity in male rats immediately after exercise was 335% of the resting value, but remained unchanged in females. To test the hypothesis that oestrogens may protect the female muscle membrane, female rats were ovariectomized at different stages of sexual maturity and exercised. A clear effect of this hormonal manipulation was observed: operated animals showed post-exercise CK elevations, depending of the age at the time of ovariectomy. Rats, ovariectomized before reaching sexual maturity, showed more damage than those ovariectomized after reaching sexual maturity.