The metabolism of [3H]estrone sulfate ([3H]E1S) was studied in normal breast tissue from 10 premenopausal women without oral contraceptives (OC), in 12 OC users and in 9 untreated postmenopausal women. [3H]E1S was converted into estrone ([3H]E1) and estradiol-17 beta ([3H]E2) by tissue samples from all three groups of women, with only minor formation of other unconjugated compounds. The rate of [3H]E2 formation was significantly higher in premenopausal women without OC than in postmenopausal women. Among premenopausal women, OC users had a significantly lower rate of total hydrolysis and of [3H]E1 formation than non-users. The rate of total hydrolysis of [3H]E1S in normal breast tissue from all three groups of women was similar to that in muscle, but the rate of [3H]E2 formation was ten times higher. Both total hydrolysis rate and rate of [3H]E2 formation were significantly lower in normal breast tissue than in breast carcinoma and in normal and neoplastic endometrium. The specific ability of normal breast tissue to convert E1S into the terminal biologically active estrogen E2 may be important for estrogenic stimulation of the breast in subjects with low circulating E2 levels. The lower rate of E1 formation in OC users may reflect an inhibitory effect of the progestagen compound in such preparations.