Rats with lesions of the mediodorsal thalamic nucleus had good retention of a preoperatively leamed odor detection task but made more errors than controls in the acquisition of a series of two-odor discrimination problems. While controls showed rapid acquisition of a learning-set, the performance of experimental rats was highly variable. Deficits in acquisition occurred on different problem for different rats and appeared unrelated to problem difficulty or serial position of the discrimination. Experimental rats also had a significant deficit in the acquisition of an odor reversal task. Error scores in both the learing-set and reversal task were related to lesion size. Small lesions confined to the rostral third of the nucleus or asymmetrical lesions which spared the central (olfactory) division of mediodorsal nucleus on one side were without effect. The results suggest that the deficit in olfactory learning produced by destruction of the mediodorsal thalamic nucleus is complex and is not adequately accounted for by stimulus novelty, problem difficulty or by poor interproblem transfer of learning.