Synaptic interactions between neurons that contained atrial natriuretic peptide (ANP), and those that contained corticotropin releasing factor (CRF) in the paraventricular nucleus (PVN) of the rat hypothalamus were examined at the electron microscopic level by pre-embedding double immunolabeling. In the double immunolabeled sections, ANP-like immunoreactive (ANP-LI) axon terminals made synapic contacts with CRF-like immunoreactive (CRF-LI) nerve cell bodies and processes. In these synapses, the occurrence of axo-dendritic and/or axo-axonic synapses was about 1.7 times that of axo-somatic synapses, and most were found in the peraventricular parvocellular subdivision of the PVN, with a few in the medial parvocellular subdivision. Conversely, in the same areas, CRF-LI terminals were rarely found to have direct contact with ANP-LI nerve cell bodies or processes. These findings suggest that, in the PVN of the rat hypothalamus, ANP-containing neurons innervate CRF-containing neurons and may regulate their secretory activity or some other functions through the synapses.