1. Direct and indirect effects of intracortical stimulattion on pyramidal tract cells were compared in the monkey and in the cat under barbiturate or chloralose anaesthesia. The hind-limb motor areas were explored, that in the monkey only within the convex part of the precentral gyrus. The intracortical stimuli were applied in the nearest vicinity of pyramidal tract cells, where antidromic spike potentials of single cells were recorded. 2. Average records of descending volleys in corticospinal tract fibres were taken from the surface of the lateral funiculus or from its dissected fascicles. The sensitivity of the recording was sufficient to detect responses in single fibres. 3. The latencies of the earliest descending volleys evoked by weak intracortical stimuli were compared with the latencies of the antidromic spike potentials of pyramidal tract cells evoked by stimulation of the lateral funiculus at a low lumbar level (same conduction distance). Only in about one third of cases these latencies were similar and compatible with a direct activation of pyramidal tract cells. In the remaining cases they indicated mono- or polysynaptic activation of pyramidal tract cells. 4. Latencies of the later components of the descending volleys indicated that they were due to indirect activation of pyramidal tract cells in practically all cases. 5. The components of the descending volleys attributable to the indirect activation of pyramidal tract cells were greatly increased when repetitive intracortical stimuli were applied instead of single ones. 6. The investigation leads to the conclusion that a weak intracortical stimulation is relatively ineffective in a direct excitation of pyramidal tract cells and that the effects of such a stimulation are mainly indirect, especially when repetitive stimuli are used.
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