Single unit activity is being recorded in the precentral cortex and brainstem of alert rhesus monkeys during the performance of several learned movements. The goal is to discern if there really are "two motor systems," one primarily for the control of distal musculature (cortex) and the other primarily for control of axial-proximal musculature (brainstem). Thus far the majority of recordings have been made in precentral cortex, and large homogeneous zones of cells related primarily to flexion or extension movements of the contralateral leg have been discovered. Such zones may be a substrate for coordination of muscular activity at a number of joints in a limb during complex movements.