The extent of intracortical arborization of individual thalamocortical axons in area 3b of the somatic sensory cortex and the degree of overlap in the cortical projections of relay cells in the ventral posterior nucleus of the thalamus were examined in macaque monkeys. Paired intracortical deposits of Fast blue (FB) and Diamidino yellow (DY) separated by 100-1500 microns were made by inserting crystals of dye into the tracks of tungsten microelectrodes used to record receptive field data on area 3b cells. Each injection gave retrograde labeling of one or more clusters of cells extending in elongated anteroposterior arrays through the ventral posterior medial (VPM) or ventral posterior lateral (VPL) nucleus. Double-labeled cells were only found when the distance between the centers of the dye deposits was less than 600 microns. With interdeposit distances greater than 600 microns, most clusters of retrogradely labeled cells had a majority of cells labeled by FB or DY. However, even with interdeposit distances of 1-1.5 mm the labeled clusters also contained significant numbers of cells labeled with the other dye. These results and an accompanying regression analysis indicate that the extent of intracortical arborization of single thalamocortical axons in area 3b is no greater than 600 microns. However, adjoining cells in the same part of the thalamic body representation can project to cortical targets as discrepant as 1.5 mm. It is proposed that the fine grain of the cortical representation depends upon inputs from the majority population of each thalamic cell cluster.(ABSTRACT TRUNCATED AT 250 WORDS)