1. Cells in area 17 of the cat visual cortex were studied with a view towards correlating receptive field properties with layering. A number of receptive field parameters were measured for all units, and nearly every unit was marked with a microlesion to determine accurately the layer in which it was found.2. Cells were defined as simple or complex by mapping with stationary stimuli, using the criteria of Hubel & Wiesel (1962). Complex cells fell into two groups: those that showed summation for increased slit length (standard complex) and those that did not (special complex).3. The simple cells were located in the deep part of layer 3, in layer 4, and in layer 6. This corresponds to the distribution of afferents from the dorsal layers of the lateral geniculate nucleus. In these cortical layers the simple cells differed primarily with respect to their receptive field size, cells in layer 4 having the smallest, layer 3 intermediate, and layer 6 the largest fields. Layer 4 was the only layer in which simple cells showed end-inhibition (a reduction in response to slits extending beyond the excitatory portion of the receptive field).4. The standard complex cells were found in all layers, but were quite scarce in layer 4. As with the simple cells, field size varied with layer: in layer 2+3 they had small to intermediate field sizes, in layer 5 intermediate, and in layer 6 very large. Layer 6 cells showed summation for slits of increased length up to very large values, and responded best when the slits were centred in the receptive field. The only standard complex cells that showed end-inhibition were those in layer 2+3, and these were similar to the layer 4 simple cells in terms of proportion of end-inhibited units and degree of end-inhibition.5. The special complex cells, originally described by Palmer & Rosenquist (1974), were found in two tiers: the upper one at the layer 3/layer 4 border and the lower one in layer 5. They were different from the standard complex cells in having a high spontaneous activity, high velocity preference, and large fields which were similar in size (at a given eccentricity) from one cell to the next. Many showed reduced response to slits of increasing length, even for slits that did not extend beyond the borders of the responsive region.6. Cells in layer 6 (the origin of the corticogeniculate projection) were antidromically activated from the lateral geniculate nucleus. The antidromically activated units included both simple and complex cells, and they had the long receptive fields characteristic of the overall population of cells in layer 6.7. The results showed that there are different types of simple and complex cells, and that cells in different layers have different properties. Taken together with their differences in site of projection, this demonstrates that the anatomical lamination pattern is reflected in functional differences between cells in different layers.