Light and electron microscopic triple immunostaining was performed on coronal vibratome sections prepared from the hypothalamus of ovariectomized (OVX) and OVX plus estrogen-treated African green monkeys (Cercopithecus aethiops). Immunoreactivity for progesterone receptors (PRs) and neuropeptide-Y (NPY) was visualized by a dark blue to black nickel diaminobenzidine reaction, while the tyrosine hydroxylase-containing perikarya were labeled with a light brown diaminobenzidine reaction. In the OVX plus estrogen-treated material, 30% of the tyrosine hydroxylase-immunoreactive neurons contained PR-immunopositive nuclei. The majority of these cells were found in the central portion of the periventricular area, and a few could be observed in the anterior hypothalamus and the arcuate and dorsomedial hypothalamic nuclei. These tyrosine hydroxylase-immunoreactive PR-containing cells were surrounded with NPY-immunoreactive axon terminals. A correlated electron microscopic analysis of the same sections revealed synaptic contacts between these NPY-immunoreactive boutons and the PR-containing tyrosine hydroxylase-immunoreactive neurons. In contrast, in the OVX animals, no PR-containing tyrosine hydroxylase-immunoreactive neurons could be detected. In these monkeys, the frequency of synaptic contacts between the NPY-immunoreactive axon terminals and tyrosine hydroxylase-immunopositive cells was similar to that in the OVX plus estrogen-treated monkeys. These observations indicate that in a population of hypothalamic dopamine cells, the presence of nuclear PRs is estrogen dependent, show that these cells are innervated by NPY axons, and suggest that these estrogen-induced PR-containing dopamine neurons are involved in mediation of the effect of NPY on hypophyseal hormone secretion, including ovarian steroid hormone-dependent LH and PRL release.