Previous studies have shown that in many different mammals there is a small but distinct projection from the supramammillary region in the caudal hypothalamus to the junctional region between the regio superior and regio inferior of the hippocampus. We have analyzed the mode of termination of this hypothalamo-hippocampal projection in the rat by electron microscopic (EM) autoradiography following injections of [3H]proline into the caudal hypothalamus. The projection is confined to the regio inferior where it is centered over the subicular end of field CA3, but also spans the adjoining region, field CA2. In our material the highest densities of labeling have been seen over the deeper part of the pyramidal cell layer and in the adjacent stratum oriens but, in addition, above background levels of labeling have been found superficial to the pyramidal cell layer in the stratum lucidum and the deeper part of the stratum radiatum. Most of the labeled synapses appear to be on the perikarya and primary dendrites of the hippocampal pyramidal cells, but some axo-spinous contacts have also been seen. All the labeled boutons contained clear, spheroidal synaptic vesicles and made asymmetric, Type I, contacts with their targets.