The distribution of immunoreactive leu-enkephalin neurons and fibers in the monkey hypothalamus, including ultrastructural localization in the paraventricular nucleus (PVN), was examined with the peroxidase-antiperoxidase immunocytochemical method. Immunoreactive leu-enkephalin cell bodies and fibers were present in the PVN, the region of the dorsal nucleus and nucleus of the anterior commissure, the dorsomedial nucleus, ventromedial nucleus, and lateral hypothalamus. Within the PVN labeled cells were found mostly in the medial parvocellular region, and a smaller proportion including some large cells was present in the lateral, and dorsolateral zones. Immunoreactive neurons contained numerous large granular vesicles (LGV) which ranged from 63 to 235 nm in size, suggesting that at least some enkephalin-containing neurons belong to the population of neurosecretory cells. Positive neurons were postsynaptic to four types of unlabeled axon terminals. Leu-enkephalin-containing fibers (some of which were myelinated) and boutons contained small clear vesicles and numerous LGV. Axon terminals made synaptic contacts with the cell bodies, primary and distal dendrites of unlabeled neurons. The findings show that enkephalin-containing neurons in the PVN integrate a variety of neuronal inputs and provide morphological evidence for the inhibiting influence of enkephalins on the firing rate of PVN neurons. It may be speculated that the effects of opioids on the release of vasopressin and other substances possibly originating from PVN neurons may be regulated in part within the nucleus by locally synapsing axons belonging to enkephalin-containing neurons.