Because peripheral aromatization is the major source of circulating estrogens in men and postmenopausal women, we studied the aromatase activity in muscle tissue from both men and postmenopausal women. To do so, the in vitro conversion of tritiated androstenedione to estrogen in homogenates of skeletal muscles obtained at autopsy was studied. Samples from lower limb muscles of both men and postmenopausal women produced estrogen, ranging from 8.5-39.8 pg/g wet wt. The conversion was almost the same as that reported for human adipose tissue, suggesting that the contributions of muscle and fat to the extraglandular production of estrogens in these subjects might be similar. This is the first direct confirmation of muscle aromatase activity and indicates the possible importance of muscle as an extragonadal source of estrogen in both men and postmenopausal women.