The fact that the lateral geniculate nucleus (LGN) of the rhesus monkey (and man) is organized in six layers, three receiving connections from each eye, has aroused considerable speculation about the significance of this triple division at the thalamic level. Le Gros Clark (7, 8) postulated that, each of the three pairs of layers is related to one of the primaries of classical trichromatic theory. Walls (13) has speculated that one pair of layers may be connected to a rod system, and the others to cones. In the present experiment, microelectrode recordings were taken from each layer of the monkey LGN while stimulating the eye with various parameters of chromatic light. We have attempted to determine directly the nature of differences among the electrical responses of the various laminae of the LGN. (PsycINFO Database Record (c) 2006 APA, all rights reserved)