Plasma dehydroepiandrosterone sulfate (DS), androstenedione (A2), estrone plus estradiol (E1+2) and estriol (E3) were measured in women treated with conjugated estrogens and controls. Plasma DS, A2 and E1+2 levels were higher in premenopausal controls than in untreated postmenopausal women; E3 levels were below the sensitivity of the assay. Estrogen conjugates, 1.25 mg or 2.5 mg daily, increased plasma E1+2 to levels similar to those in premenopausal untreated women, and E3 became readily assayable. A 0.625 mg dose increased the level of E3 without affecting E1+2; consequently, plasma E3/E1+2 ratio was elevated. At the lowest dose 16 .alpha.-hydroxylase activity, perhaps by substrate induction, is sufficient for the exogenous estrogens to be largely metabolized to E3. Estrogen conjugates had no detectable effect on plasma DS or A2.