The neuronal organization of the lateral geniculate nucleus of the prosimian primate, Galago crassicaudatus, was studied in Golgi-Kopsch-impregnated material. On the basis of cytoarchitecture, electrophysiology, and connections the nucleus is divisible into three pairs of layers--one magnocellular, one parvocellular, and one koniocellular--each part of a separate retinogeniculate and geniculostriate pathway (Itoh et al., '82; Norton and Casagrande, '82). In Macaca and Saimiri, which have equally distinct geniculate subdivisions, it has been reported that, outside of cell size, no one morphological attribute differentiates magnocellular from parvocellular neurons (Campo-Ortega et al., '68; Wong-Riley, '72; Saini and Garey, '81; Wilson and Hendrickson, '81). Results presented here are not inconsistent with this conclusion. However, when the results are analyzed from the standpoint of the collective traits that distinguish the cell groups that make up the layers, clear morphological differences are evident. Using this approach we find the following differences between presumed projection neurons and interneurons in each pair of layers. The projection neurons of the magnocellular layers, as a group, exhibit large cell bodies with radially arranged dendrites which often extend beyond laminar borders. The magnocellular interneurons are larger than their counterparts in the other layers and, like the magnocellular projection neurons, exhibit radially arranged dendrites. The former, however, also share characteristics in common with other interneurons such as relatively small somata, few proximal dendrites, and complex distal dendritic appendages. In contrast, the projection neurons and interneurons of the parvocellular layers have smaller somata and more restricted dendritic spreads than their counterparts in the magnocellular layers. Dendritic arbors of parvocellular neurons are typically oriented perpendicular to laminar borders and remain confined to their layer of origin. The koniocellular neurons represent a more diverse population but collectively are distinct in that the dendrites of almost all neurons in these layers run parallel to the layers. The fact that presumed interneurons and projection neurons in a single layer share a number of related dendritic features suggests that both groups together are responsible for the structural and, hence, functional architecture of a layer.(ABSTRACT TRUNCATED AT 400 WORDS)