The structural organization of the prefrontal cortex of the cat was investigated. In cats, just as in monkeys, the cortex of the dorsolateral surface of the prefrontal region differs sharply from the cortex of the mediobasal surface not only in its structure and level of differentiation, but also in its thalamic projections. In the cat the distribution of projections from the mediodorsal (MD) thalamic nucleus to the cortex of the anterior pole of the hemispheres exhibits a certain topography, similar in its basic features to the mode of projection of that nucleus in monkeys. In this way homologs of particular areas of MD could be identified in the prefrontal cortex of these animals. However, besides similarities, significant differences also were found. Phylogenetically older mediobasal areas predominate in the cat, whereas in monkeys the younger dorsolateral areas predominate. In the cat the parvocellular portion in the mediodorsal thalamic nucleus is absent and its homolog occupies a very small area and the cortex of the dorsolateral surface to which this portion of the nucleus projects is correspondingly very poorly developed. In monkeys, however, the parvocellular portion may occupy two thirds of the volume of MD and the dorsolateral cortex is correspondingly well developed. These observations suggest that the progressive evolution of the prefrontal cortex, connected with the higher level of behavior, has followed the line of preferential development of the dorsolateral surface and that this development is closely connected with the formation of projections of the parvocellular portion of MD in this region.