We hypothesized that estrogen administration would attenuate skeletal muscle neutrophil infiltration, indices of muscle membrane disruption, and muscle calpain activity shortly after the termination of exercise. Ovariectomized female rats were implanted with either an estogen pellet (25 mg beta-estradiol) or a placebo pellet. Two weeks postimplant, animals were killed either at rest or 1 h after running exercise (60 min at 21 m x min(-1), 12% grade). The 4 experimental groups (n = 12) used were: unexercised placebo (UP), unexercised estrogen (UE), exercised placebo (EP), and exercised estrogen (EE). Blood samples were analyzed for creatine kinase (CK) activity and estradiol content. Plantaris and gastrocnemius muscles were removed and histochemical determination of neutrophil content or biochemical determination of myeloperoxidase (MPO), glucose-6-phosphate dehydrogenase (G6PD), and calpain-like activity determined. Estrogen supplemented animals had 10-20-fold higher circulating estradiol levels than placebo animals. EP animals had significantly higher (P < 0.05) circulating CK activities than EE or unexercised animals. Muscle neutrophil concentrations were significantly (P < 0.01) elevated in EP and EE groups compared with unexercised controls, with EP muscle neutrophil levels also being over 60% greater (P < 0.05) than in EE animals. EP animals also had higher (P < 0.05) muscle MPO activities than unexercised or EE animals. Muscle G6PD activities were not significantly different between any groups. Muscle caplain-like activities were 80% higher (P < 0.01) in EP animals than EE animals with calpain-like activities in EE animals similar to unexercised groups. These results indicate that estrogen supplementation in ovariectomized rats attenuated 1-h post-exercise serum CK activities, muscle neutrophil infiltration, MPO activities, and calpain-like activities when compared with exercised, unsupplemented animals. This supports the possibility of a relationship between estrogen, calpain dependent production of neutrophil chemo-attractant peptides, and 1-h post-exercise skeletal muscle neutrophil infiltration.