Rats with electrolytic lesions of the median raphe nucleus were found to display a profound impairment in both the acquisition and retention of an eight-arm radial maze task. In contrast to neurologically intact animals, subjects with damage to the median raphe showed strong response patterning. When this patterning was disrupted by means of confinement, subjects with lesions responded at chance levels. As these results were similar to those which have been reported to follow hippocampal damage, this report provided further behavioral support for the concept of a functional relation between forebrain limbic structures and the midbrain tegmentum.