The distribution of neurons containing progesterone receptors in the brain of guinea pigs ovariectomized and primed by estradiol was investigated immunohistochemically using monoclonal antibodies to the progesterone receptor. We found that the picric acid paraformaldehyde perfusion provided satisfactory conditions of fixation for visualizing the progesterone receptor in sections of frozen tissue. Among the different techniques of immunocytochemical detection used, the indirect antibody peroxidase-antiperoxidase method gave the best results. The immunostained neurons were mainly located in two specific regions of the hypothalamus: the preoptic area and the mediobasal hypothalamus. Within the preoptic region, the majority of immunoreactive cells were present in the nucleus preopticus periventricularis particularly at the level of the pars suprachiasmatica. Within the mediobasal hypothalamus, immunostained neurons were found in the nucleus periventricularis, arcuatus, ventromedialis and premamillaris. Differences in the intensity of immunoreactivity appeared from one region to another. A marked cellular heterogeneity was observed: in each neuroanatomical structure, labeled cells alternated with non-labeled cells. The receptor, even in absence of progesterone, was localized in the nucleus. The nucleolus was not stained and only neurons were labeled. There was no progesterone receptor immunoreactivity in other regions of the brain, especially in the amygdala, hippocampus and cortex where biochemical studies have shown the presence of a non-estrogen regulated protein binding the progestin [3H]R 5020. Control experiments with antibody pretreated with purified progesterone receptor or with mouse receptor-unrelated monoclonal antibody did not show fluorescent or immunoreactive cells.