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We measured tightly bound nuclear estrogen receptors (ER) in sex skin biopsies obtained from pig-tailed macaques (Macaca nemestrina) which were previously ovariectomized and treated with an estradiol-progesterone regimen. Incubation of fresh tissue slices with a saturating concentration of [3H]estradiol (E2) was done to determine the capacity of nuclear acceptor sites to bind activated ER with high affinity. The radiolabeled ER was extracted from nuclei with 0.5 M KCl, complexed with an anti-ER monoclonal antibody, and quantitated by analysis on sucrose gradients. Even though serum E2 levels were unchanged, 7 and 14 days of sequential progesterone (P) treatment decreased ER amounts below those found after 7, 14 and 21-23 days of E2 treatment. ER regulation in sex skin of this species is similar to that found in macaque reproductive tract; P suppresses ER levels even in the presence of continuous E2. The tissue responses of sex skin to the hormone treatments correlated well with the measured fluctuations of tightly bound nuclear ER, which suggests the functional significance of this ER component.