The avian thalamic ventral lateral geniculate nucleus (GLv) was studied by light microscopic techniques in order to understand its anatomy, neuronal composition, and the nature of its retinal and tectal afferents. The avian GLv is of considerable interest because physiological experiments show that it is the brain structure with the highest percentage of color-opponent responses (Maturana and Varela, '82). We used adult pigeons and quail for the present study. With Nissl techniques a predominance of medium-size neurons (58%) constitute the GLv. The shape, size, and orientation of the different neurons is highly variable throughout the GLv. With the Golgi methods, 5 classes of neurons are distinguished: I and IV (large), II (medium-size), III and V (small). Some class IV large neurons have bifurcated axons; no axons were distinguished on the small neurons. Optic fibers penetrating the GLv are often collateral branches of retinal axons that continue elsewhere. Fink-Heimer methods show that retinal axon terminals end around large and medium-size neurons and also reach the internal lamina of the GLv. HRP tracing shows that the large and medium-size neurons of the GLv project to the optic tectum. On the basis of comparisons between the cytoarchitecture of the GLv described here and the physiological findings previously reported (Maturana and Varela, '82; Pateromichelakis, '79), we suggest that: (1) large GLv neurons are the color-opponent units, (2) medium-size neurons are the movement-sensitive units, and (3) small neurons are either interneurons (local circuit neurons), or they might project to the area pretectalis or to some other GLv projection region not yet described.