The performance of rats trained in a radial arm maze was tested before and after chemical lesions of the thalamic mediodorsal nucleus or the ventral tegmental area. Their behavior was compared with that of a sham-operated control group. Lesions were produced with ibotenic acid, a compound that selectively destroys neurons while apparently leaving fibers of passage intact. Results revealed no intergroup difference in the number of errors performed when all six trials were given in one session without interruption. The group with mediodorsal lesions made, however, significantly more errors than either of the other two groups when a delay of 1 hr was interposed between the first four and the last two trials. Furthermore, these rats differed from rats of the other groups in the number of sessions necessary to reach criterion, in the time needed to finish a session, and in the directness with which a goal was approached. Rats with lesions of the ventral tegmental area did not differ from rats of the sham-operated control group in any of the measures taken. It is suggested that the deficits of rats with mediodorsal lesions resemble qualitatively those found in human patients with lesions of the mediodorsal nucleus.