Plasma levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and estradiol (E2) were measured serially in 11 premenopausal patients before and after hysterectomy with bilateral salpingo-oophorectomy. One week after operation an incremental dosage regimen of conjugated estrogens (CEE) in tablet form was commenced on a basis of two weeks with therapy (0.3, 0.625, 1.25, and finally 2.5 mg.), with each dose interspersed by two weeks without therapy. FSH, LH and E2 levels were measured at the end of each period with and without therapy. E2 levels fell within 24 hours of operation while FSH and LH levels rose gradually. CEE therapy produced an elevation of E2, but circulating concentrations comparable to the premenopausal values were only maintained during the dosage periods of 0.625 and 1.25 mg. of CEE. In only one instance did CEE succeed in reducing FSH to premenopausal levels, and that was at a dosage of 2.5 mg., in which instance the E2 level was higher than the premenopausal value. LH was never reduced to a premenopausal level. Thus, the data indicate that CEE alone in dosages up to 2.5 mg. per day was unable to reproduce in postmenopausal women the gonadotropin and E2 blood serum levels shown to exist prior to oophorectomy. Usual CEE treatment after menopause, therefore, in itself does not represent physiologic "hormone replacement therapy," if defined as the dosage required to maintain premenopausal circulating concentrations of reproductive hormones.