To investigate the connections between the major limbic structures and the nucleus accumbens septi or the nucleus fundus striati, stereotaxic surgeries were performed. One group of the rats were electrically lesioned in the mamillary body, and the other group were extirpated their hippocampal formation. Careful study of both nuclei following each surgery showed the following results. 1. Nerve terminals of mamillo-accumbens tract were synapsed to the dendrite of nucleus accumbens cell, whereas the neuronal type of accumbens-mamillary tract was aspiny cell. 2. Nerve terminals of mamillo-fundus tract were synapsed to the spines of fundus striati cells. Fundus-mamillary tract cells were not confirmed. 3. Nerve terminals of hippocampo-accumbens terminals were synapsed to the dendrites and spines of nucleus accumbens cells, whereas the neuronal type of accumbens-hippocampal tract was spiny one. 4. Nerve terminals of hippocampo-fundus tract were synapsed to the spines of fundus striati cells, whereas the neuronal types of fundus-hippocampal tract was aspiny one. 5. From the results, it was concluded that both of the nucleus accumbens septi and the fundus striati have connections with the mamillary body and the hippocampus. But nucleus accumbens septi has apparently more intimate relationship with major limbic structures.