Estrogenic stimulation of the ventromedial hypothalamus is sufficient to prime progesterone-facilitated estrous behavior in ovariectomized rats. To determine precisely the site(s) of estrogenic stimulation and the locus of its priming action on estrous behavior, we used steroid autoradiographic methods to assess the diffusion of [3H]estradiol ([3H]E2) from behaviorally effective implants diluted 1:300 with cholesterol. Ovariectomized rats received [3H]E2-cholesterol implants aimed at the ventromedial hypothalamic nucleus (VMN). Females were tested twice for feminine sexual behavior after stereotaxic surgery. They received progesterone on the day of behavioral testing. Animals were killed on the day after the second behavior test, cannulae were removed, and the brains were frozen rapidly and processed for autoradiography. Sections of 6-micron thickness from the preoptic region through the posterior hypothalamus were exposed to the nuclear emulsion-coated slides for varying periods of up to 12 months. Secti...