In adult cats, the pyramidal tract response to stimulation of motor cortex at the surface and at various depths, using gross and fine stimulating electrode combinations, was analyzed, with particular emphasis upon threshold and latency determinations. The effects of polarization upon the responses of units recorded locally in motor cortex were also determined. When the motor cortex is stimulated by the usual monopolar or bipolar electrodes applied to the pial surface, the D-wave results from primary excitation of PT axons in the deep cortex or subcortical white matter. Some more slowly conducting axons that are also directly stimulated probably contribute to the I-waves. The I-wave complex is predominantly the result of excitation of cortical afferent fibers in the deep cortex or subcortical white matter. The cortical response associated with projected PT volleys is therefore analogous to the cortical response evoked by excitation of specific thalamo-cortical radiations. The implications of these conclusions for motor cortex and other central nervous system stimulation are discussed.