The antiestrogens MER-25, CI-628, and nafoxidine inhibit the uptake of [3H]-estradiol in whole homogenates and isolated cell nuclei tissues and the pituitary, and inhibit estradiol-induced female sexual behavior. The antiestrogens were injected intraperitoneally 2 h prior to an intravenous injection of [3H]estradiol, and the animals were killed 2 h after the estradiol. CI-628 reduces radioactivity in whole homogenates and isolated cell nuclei of cerebral cortex, hypothalamus, preoptic area -septum and pituitary. Nafoxidine reduces uptake in cell nuclei of the hypothalamus, preoptic area-septum and pituitary. In this paradigm, MER-25 inhibited uptake only in the pituitary. In the analogous behavioral experiments, with antiestrogens injected 2 h prior to an intravenous injection of unesterified estradiol, CI-628 and nafoxidine totally inhibited lordosis responding. MER-25 shows no inhibition of behavior in this paradigm. However, when MER-25 is injected 12 h prior to the estradiol, it inhibits retention of [3H]estradiol at 2 h in brain and pituitary cell nuclei, and lordosis responding is also inhibited. Additionally, the antiestrogens can apparently displace previously bound [3H]estrdiol. When the antiestrogens are injected 2 h prior to an injection of [3H]estradiol, MER-25, CI-628 and nafoxidine all show greater inhibition of nuclear estradiol retention at 12 h after the [3H]estradiol injection than 2 h. Analogously, when CI-628 is injected 2 h after an intravenous injection of [3H]estradiol, it displaces most of the radioactivity present in hypothalamic-preoptic area nuclei at 12 h after the estradiol injection. These results indicate that antiestrogens can prevent or reverse the nuclear concentration of estradiol in brain cells and are consistent with a role of the cell nucleus in the induction of estrous behavior by estradiol.