All of the progesterone receptor-containing cells of the monkey hypothalamus are GABAergic. The aim of this study was to further characterize these GABAergic progesterone receptor-containing neurons based on their calbindin or parvalbumin content. These calcium-binding proteins are characteristic markers of different populations of GABAergic neurons in the central nervous system. Double-immunolabeling for progesterone receptor and either calbindin or parvalbumin was performed on hypothalamic Vibratome sections of estrogen primed African green monkeys (Cercopithecus aethiops). Progesterone receptor-containing calbindin-immunoreactive neurons were observed in the ventromedial and periventricular areas of the hypothalamus. Forty-one per cent of the progesterone receptor-containing cells in this area were calbindin immunopositive. No double-immunolabeled neurons could be detected in the infundibular (arcuate) nucleus. In tissue double-immunolabeled for progesterone receptor and parvalbumin, none of the progesterone receptor-containing neurons exhibited immunoreactivity for parvalbumin. Electron microscopic double-immunostaining for progesterone receptor and calbindin confirmed the light microscopic results. Furthermore, a large number of asymmetric synaptic contacts were observed on the calbindin-immunoreactive neurons. These observations demonstrate that progesterone receptor-containing cells in the monkey mediobasal hypothalamus consist of at least two different types of GABA neurons, and indicate that progesterone receptor-containing calbindin cells may be postsynaptic targets of excitatory fibers.