Infusions of isotopically labeled [3H] androstenedione with measurement of [3H] estrone in normal breast and breast tumor tissue have been carried out in an attempt to determine the contribution that aromatization makes to the estrogen content of breast tissues. After infusion of [3H] androstenedione for 12h there was significant uptake of this steroid by normal breast and breast tumors. [3H] Estrone was detected in all samples of normal breast tissue examined so far but not in all tumors. Aromatase activity when measured in vitro was found to be higher in breast tumors than in fat next to the tumor or normal breast fat. Studies in which we have examined the effect of epidermal growth factor on aromatase activity in cultured breast adipose tissue suggests that the response may be influenced by a subject's menopausal status. Results from these preliminary studies suggest that the aromatization of androgens may make a significant contribution towards the estrogen content of some breast tumors and that growth factors may also be involved in regulating aromatase activity.