The effects of tamoxifen and 17 beta-estradiol on the levels of FSH, PRL, and pregnancy zone protein were compared in two groups of postmenopausal women. Seventeen women with breast cancer were treated with tamoxifen (20 mg, twice a day). Fourteen women with climacteric complaints were given 17 beta-estradiol (2 mg, daily). A close parallelism between the effects of 17 beta-estradiol and the antiestrogen was obtained in all three markers studied. The percent decreases in FSH after 1 month were 29 and 44 and, after 3 months, 26 and 34 in the tamoxifen and estradiol groups, respectively. The decreases in PRL after 1 and 3 months of treatment with tamoxifen were 36% and 71%, and 19% and 31% after treatment with estradiol. Both treatments increased PZP serum levels, tamoxifen by 32% and 44% and estradiol by 55% and 70% after 1 and 3 months. Thus, tamoxifen was found to exert weak estrogenic effects in postmenopausal women.