The acquisition of a spatial alternation task was tested in five groups of adult cats. Two groups were used for control; one of them consisted of nonlesioned cats, the other of cats with chemical lesions (ibotenic acid) of the ventral tegmental area. The other three groups of cats received chemical lesions of the mediodorsal nucleus of the thalamus, the mamillary bodies, or of both structures together. Cats with lesions of the ventral tegmental area were non-significantly impaired in comparison to non-lesioned cats in the acquisition of the task. Cats with lesions of the mediodorsal nucleus, the mamillary bodies, or with combined lesions of both structures were significantly impaired, compared to the two control groups; among each other, however, their performance rates were similar. As none of the cats with single or combined lesions of the mamillary and the mediodorsal nuclei manifested observable abnormalities with respect to motor, motivational, or emotional behavior during their testing period, it is concluded that their--compared to the other two groups--inferior performance was due to a memory defect. As all three kinds of lesions resulted in a similar defect, it is suggested that the mamillary and the mediodorsal nuclei transmit information to one or more common target area(s), which is (are) dependent on information from either nucleus to operate effectively.