Light microscopic studies of vibratome sections, which were double-immunostained for corticotropin-releasing factor (CRF) and for somatostatin (SS), suggested the presence of reciprocal synaptic relations between neurons containing immunoreactive (ir) CRF and those containing ir SS in the parvocellular paraventricular nucleus (parvo-PVN) and in the anterior periventricular area (APV) of the rat hypothalamus. In the sections the peptides included in neuronal fibers were labeled black with silver-gold particles, and the peptides included in neuronal cell bodies were labeled brown with diaminobenzidine (DAB). Thereby the brown cell bodies appeared to be surrounded by several black nerve terminals. In electron microscopic studies, the labeling was mostly performed in reverse fashion, because of the convenience for observing the ultrastructural details of the nerve terminals. The neuroplasm of the postsynaptic perikarya and dendrites was labeled with gold-coated silver grains, while the presynaptic axonal terminals were shown with scattered DAB particles. Granular structures in the perikarya or axonal terminals were labeled distinctively. The synaptic morphology appeared to be either symmetric or asymmetric connections. Then we found synaptic connections between presynaptic ir SS containing fiber terminals and postsynaptic ir CRF containing perikarya in the parvo-PVN, and those ir CRF containing fiber terminals and ir SS containing perikarya in the APV. The existence of such a reciprocal association between CRF and SS neurons may suggest that these neuronal systems intervene among different functional systems in the hypothalamus.