It is known that microstimulation in the depth of the motor cortex can produce contraction of a single muscle. To study the neuronal mechanisms, the following experiments were carried out using the monkey. The recording electrode was positioned within the hindlimb area of the motor cortex in such a way that spike potentials of 3-4 PT cells could be simultaneously recorded in response to stimulation of the lateral funiculus. Spinal segments L6 through S1 were then systematically explored and thresholds for excitation of axonal branches of each PT cell were determined at regular intervals in reference to the location of identified motoneuron pools. The positions of all stimulating sites were reconstructed by histological examination. The intranspinal branching of 13 PT cells in 4 groups has been analyzed. A group of PT cells which could be recorded simultaneously by the same electrode sent axon branches to various parts of the lumbar cord. Each PT neuron sent branches to more than one motoneuron pool and some of them also branched to intermediate regions of the gray matter. In addition to these divergent projections from a group of PT neurons, there was usually a small area in the ventral horn, often one motoneuron pool, where all or the majority of the group of neurons sent at least one of the their branches. Our results thus suggest that a group of adjacent PT cells exert influences on various species of motoneurons, but that this influence is the strongest on motoneurons of one species. (Asanuma, H., Hongo, T., Jankowska, E., Marcus, S., and Zarzecki, P., manuscript in press). BIBLIOGRAPHIC REFERENCES: Shinoda, Y., Arnold, A.P., and Asanuma, H.: Spinal branching of corticospinal axons in the cat. Exp. Brain Res., 26(1976) 215-234. Asanuma, H., Arnold, A., and Zarzecki, P.: Further study on the excitation of pyramidal tract cells by intracortical microstimulation. Exp. Brain Res., 26(1976) 443-461.