The effects of 2 weeks of oestrogen (40 microg kg BW-1 beta-estradiol 3-benzoate) injection on 24 h post-exercise myeloperoxidase (MPO) activities were determined in plantaris and soleus muscles and liver of sexually mature male and female rats. The treadmill running protocol (45-60 min, at 28 m min-1, 15% grade) induced significant elevations in muscle MPO activities 24 h post-exercise in male rats, while prior oestrogen administration to male rats eliminated the post-exercise elevations in muscle MPO activities. Female rats experienced no significant post-exercise elevations in muscle MPO activities. Hence oestrogen administration to male rats attenuated post-exercise muscle MPO activities to levels found in female animals. Liver MPO activities were not significantly affected by exercise, gender or oestrogen administration. Oestrogen may be a factor in diminishing 24 h post-exercise skeletal muscle leukocyte infiltration and inflammatory response in both male and female muscle.