An experiment was performed in order to determine the efferent projections of the ventral hippocampus and to compare its projection system with that of the dorsal hippocampus in the cat.Lesions were placed stereotaxically in either dorsal or ventral hippocampus in 25 cats. Animals were sacrificed after post-operative survival periods of 4 to 8 days. The Fink-Heimer and Nauta-Gygax techniques were used to trace degenerating axons and their terminals from ventral and dorsal hippocampus.The results demonstrated: (1) a differential distribution of fibers from the ventral and dorsal hippocampus to the septum; i.e., ventral hippocampal fibers terminate in massive numbers throughout the entire extent of the lateral septum and nucleus accumbens; dorsal hippocampal fibers terminate in lesser amounts in the medial septal nucleus; fibers from the posterior crus of the ventral hippocampus project to the midlateral septum; (2) both dorsal and ventral hippocampal fibers project similarly to the diagonal band of Broca, anterior ventral nucleus of thalamus, lateral hypothalamus, and mammillary bodies. The rostral half of the lateral hypothalamus receives fibers principally of ventral hippocampal origin whereas the posterior half of the lateral hypothalamus and mammillary bodies receive fibers of dorsal hippocampal origin; (3) the paraventricular nucleus of the hypothalamus, posterior hypothalamic nucleus, and nucleus of Darkshewitsch receive a few fibers solely from the dorsal hippocampus; (4) a second efferent pathway from the ventral hippocampus was traced from the alveus to pyriform cortex.