Single cell activities were recorded from both the supplementary and precentral motor areas (SMA and PCM) of individual monkeys in order to compare their relation to performance. In the first series of experiments, monkeys were trained to perform a simple movement of key pressing in response to sensory signals of three different modalities: visual, auditory and somatosensory. It was found that the intensity of the movement associated neuronal activity was smaller in SMA than in PCM, and that onsets of neuronal activity in SMA were not as well correlated with the animal's movement onsets as those in PCM. These findings suggest that SMA is more remote from the peripheral motor apparatus than PCM. On the other hand, SMA seems to be closer to visual and auditory inputs. In the second series of experiments, monkeys were trained in a behavioural paradigm where instructions required them to predetermine their motor response to a signal of a particular sensory modality. In one situation, an instruction required the animal to be prepared to start a movement promptly in response to a forthcoming tone burst but to remain motionless if the signal was vibrotactile. In a second situation, a different instructional signal required the animal to be prepared to execute the movement if the vibrotactile, but not the tone burst, was presented. Striking differences in the instruction-induced activities in the two motor areas were found, indicating that SMA plays a more important role than PCM in a preparatory process leading to correct initiation on suppression of movement performance.