Rats were trained preoperatively on contingently reinforced alternation in a T-maze. Then different matched groups of rats received lesions in the prelimbic cortex, mediodorsal thalamus, posterodorsal septum (aimed at transecting the precommissural fornix), and control operations (no brain lesions). Following a 2-week recovery period the rats were retested in the T-maze for retention of delayed alternation. Control rats were unaffected by the control operations and the testing hiatus of the recovery period. Rats with lesions in the prelimbic cortex performed at chance levels on the first postoperative session as did rats with posterodorsal septal lesions, but both groups recovered with continued experience, i.e., they could relearn the task. Rats with lesions in mediodorsal thalamus were only slightly affected by the lesions. The results suggest that a restricted field in the medial pregenual cortex, the prelimbic area, is critically involved in T-maze alternation. However, the data also suggest that a major subcortical source of afferents to prelimbic cortex, the mediodorsal nucleus of the thalamus, is not crucial for retention of contingently reinforced T-maze delayed alternation following interference with septo-hippocampal circuitry is consistent with data previously reported.