The number of synapses per unit volume of tissue (NV) has been estimated in individual laminae of the binocular and monocular regions of area 17 in six adult cats by using a method of size-frequency distribution. Separate estimates were obtained for RA synapses (containing round vesicles associated with asymmetric membrane differentiations) and for FS synapses (containing flat vesicles associated with symmetric membrane differentiations). For the total cortical thickness, the NV of all synapses is not statistically different between binocular (286 million per mm1(3] and monocular (281 million) regions, nor is it different between the two regions for any of the laminae. Eighty-four percent of synapses are of the RA type. Of those, 79% are found on dendritic spines, 21% on dendritic trunks, 0.1% on somata. FS synapses represent 16% of the total, with 31% of them on spines, 62% on dendritic trunks, and 7% on somata. The ratio of RA to FS synapses is kept relatively constant throughout the layers. A two-way analysis of variance shows no difference in the NV of either RA or FS synapses in the two regions nor in the NV or RA synapses between cats. It does, however, clearly demonstrate (p less than 0.001) interindividual differences for FS synapses. These variations between individual cats may be due to differences in age, breed, or environmental factors. In contrast to the relative uniformity of the NV of synapses between regions, the number of each type under 1 mm2 of cortical surface is 33% higher in the binocular region. This is due mainly to the greater thickness of the binocular region.