Plasma dehydroepiandrosterone sulfate (DS), androstencdione (A2), estrone plus estradiol (E1+2) and estriol (E3) were measured in women treated with conjugated estrogens and controls. Plasma DS, A2, and E,+2 levels were higher in the premenopausal controls than in the 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 Ea became readily assayable. A 0.625-mg dose increased the level of E3 without affecting E1+2; consequently, the plasma Ea/E1+2 ratio was elevated. It is concluded that at the lowest dose 16 a-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.