Rats with electrocoagulation of the ventral mesencephalic tegmentum surrounding the interpeduncular nucleus were compared to control rats in various behavioral testing situations such as alternation behavior, spatial discrimination learning, and reversal in a T-maze. Lesioned animals showed disturbances of alternation behavior, impairments in spatial discrimination, acquisition, and retention, and great difficulty in reversing previously learned habits. These defects were not imputable to a deficit in memory processes. They indicate impairment of attentional mechanisms. It appeared that the lesion of this part of the limbic midbrain area provoked a behavioral syndrome similar to that resulting from limbic forebrain lesions. © 1979 Academic Press, Inc.