Thalamic afferents to the prefrontal cortex in the cat were traced with horseradish peroxidase (HRP). Following injections of HPR into the medial prefrontal cortex and the ventral portion of the lateral prefrontal cortex, a large number of labeled cells were found in the dorsomedial nucleus of the ipsilateral thalamus, particularly its medial and dorsal parts. Labeled cells were also detected in the submedial and ventral medial nuclei, more in the former. Very few cells were labeled in the medial part of the medial pulvinar nucleus and in the paratenial and medial central nuclei. Occasional labeled cells were seen in the paracentral, rhomboid and reuniens nuclei. Injections of HRP into the dorsal part of the lateral prefrontal cortex, including the medial bank of the presylvian sulcus, resulted in heavy labeling of cells particularly in the lateral and ventral parts of the ipsilateral dorsomedial nucleus. Some labeled cells were found in the ventral medial nucleus, and a lesser number in the submedial nucleus. Very few cells were labeled in the paratenial nucleus and in the medial part of the medial pulvinar nucleus. Occasional labeled cells were detected in the paracentral, lateral central and reuniens nuclei. The above results reveal that with regard to the thalamic afferents, appreciable differences between the dorsolateral and ventromedial prefrontal cortices were noted.