By using a combination of monoclonal antibodies to progestin receptors and a double-bridge peroxidase-antiperoxidase technique, a sensitive immunocytochemical method was developed for visualizing progestin receptor immunoreactivity (PR-IR) in brains of estrogen-primed guinea pigs. PR-IR neurons were observed throughout the hypothalamus and preoptic area. They were seen in largest numbers in the arcuate nucleus, periventricular preoptic regions, medial preoptic nucleus, medial preoptic area and in the ventrolateral area of the hypothalamus. Lower numbers of PR-IR positive cells were detected in the bed nucleus of stria terminalis, paraventricular nucleus and lateral hypothalamus with scattered cells seen throughout the hypothalamus and preoptic area. The PR-IR was mostly intranuclear with lighter staining in neuronal processes. Electron microscopy confirmed the predominantly intranuclear localization and further demonstrated that the reaction product was dispersed throughout the nucleus, but not associated with the nucleolus. Few PR-IR cells were observed in the absence of estradiol priming, and the reaction product was much lighter than in the presence of estradiol. Progesterone injection was without apparent effect on intensity of the PR-IR.