The relationship of the strength of interaction between precentral cortical neurons and their distance of separation during active reaching movements was studied in adult primates. Chronic unit recording experiments with two independent microelectrodes were performed in the left precentral forearm area of monkeys trained to execute reaching movements with the right arm in response to a visual cue. Neurons were identified by the joint actions produced by intracortical microstimulation. Cross-correlation analysis was employed to assess the strength of interaction between units. Unit pairs which exhibited the highest strength were recorded by the same electrode. For unit pairs derived from separate electrodes, the incidence and strength of interaction fell as the separation between the units was increased. Neurons identified by intracortical microstimulation as controlling the same or contiguous joints tended to interact with each other with much higher probability than did those neurons identified as controlling non-contiguous joints. When the direction of flow of information was assessed, these was a preferential flow from neurons controlling proximal joints to those controlling distal ones. These results are consistent with recent findings of tight kinematic coupling between contiguous joints and the observation of proximal-to-distal sequence of activation at the neuronal and electromyogram levels during voluntary movement.