1. Monkeys were trained to 1) hold a handle in a central zone midway between "push" and "pull" while awaiting 2) an instruction telling them how to respond to a subsequent 3) perturbation, which triggered the instructed movement and was followed by 4) a reward if the movement was correct. 2. There were two sorts of instructions: push and pull. When the pull instruction had preceded the perturbation, the monkey responded to the perturbation by pulling, whereas after a push instruction, the monkey responded to the perturbation by pushing. 3. Recordings in pre- and postcentral sensorimotor cortex revealed instruction-induced changes of neuronal activity during the period intervening between the instruction and the perturbation-triggered movement. Effects of the instruction were differential depending on which of the two instructions was given, such differential responses to the instruction being detected in 61% of precentral pyramidal tract neurons (PTNs), 44% of precentral non-PTNs, and 11% of postcentral neurons. 4. Since motor cortex PTN axons end on alpha and gamma motoneurons and on interneurons of the spinal cord, changes of PTN activity with "intention" or "motor set" provide a mechanism for suprasegmental control and presetting of spinal cord reflex excitability specific to the nature of an impending movement.