The descending connections of the mammillary nuclei of the rat were studied using the autoradiographic technique to trace anterograde axonal connections. Injections of tritiated leucine or tritiated proline were stereotaxically aimed at either the medial or lateral mammillary nucleus. Label in the medial and lateral mammillary nuclei was rated as either high or low. With a rating of high in both medial and lateral mammillary nuclei (11 animals) three major projection sites were seen: the dorsal tegmental nucleus (DTN), the ventral tegmental nucleus (VTN) and a pontine region which included both the pontine grey and the tegmental reticular nucleus. In the nine animals in which the medial mammillary nucleus was rated high but the lateral mammillary nucleus was rated low, no label was present in DTN, although the other two regions were labeled. The one animal in which the lateral mammillary nucleus was high but the medial was low, had label in all three major projection sites; since fibers destined for the DTN pass through the VTN, it could not be determined whether or not lateral mammillary efferents were in synaptic contact with the VTN. Labeled fibers were present throughout a wide dorsal-ventral extenf of the mesencephalon; these fibers are components of the mammilotegmental system and not part of the mammillary peduncle.