The localization of serotonin and non-serotonin-containing cell bodies in the interpeduncular nucleus of the rat that project to the hippocampal formation was studied using the technique of retrograde tracing of Granular Blue and immunohistochemistry on the same sections. The results indicate that the caudal magnocellular subnucleus (pars dorsalis magnocellularis) and, to a lesser extent, the caudal part of the lateral subnucleus (par lateralis) of the interpeduncular nucleus send serotonin as well as non-serotonin fibers to the ventral hippocampus.