The purpose of the present study was to investigate the mechanisms by which estrogen production arises in the human female following the menopause. The possibilities considered were a) glandular secretion of estrone (E1) or estradiol (E2), b) the peripheral aromatization of circulating androstenedione (A), or c) the aromatization of other plasma precursors of estrone or estradiol. Studies were carried out in 6 postmenopausal women in which the following were measured: the plasma production rate of A (PRPA), the fraction of plasma androstenedione which was converted to estrone as measured in urine, (PRpA) and in blood, [ρ]AE1BU, and the total estrone production rate as measured by the urinary method (PRE1). The mean PRPA, 1.75 mg/24 hr, was about one-half of that observed in premenopausal women and arose principally from the adrenal glands. The mean transfer constant, [ρ]AE1BB value was 0.027, which is about twice the value obtained in premenopausal women, suggesting an increase in conversion of A to E1 with age. The results suggests that the principal estrogen formed in postmenopausal women is estrone and that it is derived by aromatization of plasma A. Further, essentially all estrone production in these women was accounted for by this process, since the mean quantity of E1 derived from A, PRE1A, 45.4 μg/24 hr was virtually the same as the mean PRE1, 46.3 μg/24 hr. It is therefore likely that neither adrenal nor ovarian secretion of estrone or estradiol contributes significantly to total estrogen production in postmenopausal women.