Physiological studies of the lateral geniculate nucleus (LGN) have revealed three classes of relay neurons, called X, Y, and W cells in carnivores and parvocellular (P), magnocellular (M), and koniocellular (K) in primates. The homological relationships among these cell classes and how receptive field (RF) properties of these cells compare to LGN cells in other mammals are poorly understood. To address these questions, we have characterized RF properties and laminar organization in LGN of a highly visual diurnal rodent, the gray squirrel, under isoflurane anesthesia. We identified three classes of LGN cells. One class found in layers 1 and 2 showed sustained, reliable firing, center-surround organization, and was almost exclusively linear in spatial summation. Another class, found in layer 3, showed short response latencies, transient and reliable firing, center-surround organization, and could show either linear (76%) or non-linear (24%) spatial summation. A third, heterogeneous class found throughout the LGN but primarily in layer 3 showed highly variable responses, a variety of response latencies, and could show either center-surround or non-center-surround receptive field organization and either linear (77%) or non-linear (23%) spatial summation. RF sizes of all cell classes showed little dependence on eccentricity, and all of these classes showed low contrast gains. When compared with LGN cells in other mammals, our data are consistent with the idea that all mammals contain three basic classes of LGN neurons, one showing reliable, sustained responses and center-surround organization (X or P), another showing transient but reliable responses, short latencies, and center-surround organization (Y or M), and a third, highly variable and heterogeneous class of cells (W or K). Other properties such as dependence of receptive field size on eccentricity, linearity of spatial summation, and contrast gain appear to vary from species to species.