Using electron immunocytochemical method, the ultrastructural distribution and the synaptic connections of CCK-containing neurons in the paraventricular nucleus (PVN) of the rat were studied. The results showed that the CCK-like immunoreactive products located in farge granular vesicles, cytoplasmic matrix, at the periphery of small clear vesicles, rough endoplasmic reticulum and the membrane of mitochondria. The CCK-positive nerve cell bodies were large or small in size and distributed mainly in the medial part of the PVN, subependymal region and the vicinity of capillaries. Some of them as postsynaptic elements formed axosomatic synapses with CCK-negative axonal terminals. The CCK-positive dendrites and axons situated everywhere in the PVN. Some of them as postsynaptic elements formed axodendritic and axoaxonic synapses with CCK-negative structures. Some CCK-positive axonal endings surrounded the capillaries. Other CCK axonal terminals as presynaptic elements formed axosomatic, axondendritic and axo-axonic synapses with CCK-negative structures, respectively. In addition, we have first found that the CCK-positive dendrites penetrated ependyma and contacted directly with the cerebrospinal fluid in third ventricle, the CCK-positive axons traveled in the cavity of third ventricle near the ependyma. The above mentioned results suggested: (1) the soma, dendrite and axon of the CCK-containing neurons and CCK-negetive neurons in the PVN might form local neuronal circuit; (2) the neuron vessel circuit might be established between CCK-containing neurons and the blood vessels in the PVN; (3) the CSFcontacting neurons in the PVN may participate in forming brain-cerebrospinal fluid neurohumoral circuit and regulate functional activity of distal target area through the CSF pathway.