The thalamic afferents to cortical areas 4 and 6 in the cat have been studied using retrograde axonal transport of horseradish peroxidase. Areas 4 and 6 receive projections mostly from the ventral lateral (VL) and ventral anterior nuclei. Area 4 gamma receives fibers from the central, ventral and ventrolateral parts of the VL, area 4 delta from the lateral part, area 4sfu from the dorsal and dorsolateral parts, and area 4fu from the dorsal and dorsomedial parts. Area 6a beta receives fibers from the ventromedial part of the VL, area 6a alpha from the medial part, and area 6iffu from the dorsomedial part. Thus, the VL projects to areas 4 and 6 in a topical manner. The ventrodorsal axis of the VL is projected along the anteroposterior axis of areas 4 and 6. In addition, the VL projects topically to these areas also in the mediolateral direction. A considerable number of neurons in the ventral anterior nucleus (VA) send axons to areas 4 and 6. Area 4 receives fibers from the ventrolateral part of the VA, while area 6 receives fibers from its dorsomedial part. These projections are organized topically; the ventrodorsal axis of the VA is represented by the anteroposterior axis of areas 4 and 6. In addition to the VL and VA, areas 4 and 6 receive a fair number of fibers from the lateral central nucleus, and a few fibers from the paracentral, parafascicular and medial central nuclei and from the dorsolateral part of the posteromedial ventral nucleus. Area 4 receives only few fibers from the centre median and limitans nuclei. Area 6 receives a few fibers from the submedial and ventral medial nuclei.