This report compares neuronal activity in the supplementary motor area (SMA) and the precentral motor cortex (PCM) in response to auditory and vibrotactile signals that required a monkey either to start a key-press movement or to refrain from initiating such a movement. Confirming previous reports (3, 9), a vibrotactile stimulus that triggered movement gave rise to two phases of neuronal activity in PCM neurons: a short-latency response time-locked to the occurrence of the vibrotactile stimulus, and a response related to the time of onset of the movement. When the animal was required to refrain from moving in response to the vibrotactile signal, the short-latency response was often attenuated and there was rarely any later activity. There was no attenuation of the short-latency response to the nontriggering vibrotactile stimulus in the anterior part of the postcentral somatosensory cortex. As reported previously (23), short-latency stimulus-locked responses of SMA neurons to a vibrotactile signals were less frequent and the magnitude of the responses was smaller than in the PCM. However, the properties of the later-occurring responses of SMA neurons were often different from those of PCM neurons. Many SMA neurons responded to both the triggering and nontriggering vibrotactile signals. Twenty-nine SMA neurons responded to the nontriggering signal only and not to the movement-triggering signal. Most of the PCM neurons were active after the auditory signal only when the signal was a trigger to start the key-press movement; three neurons exhibited a slight activity increase after the nontriggering auditory signal. In contrast, a number of SMA neurons responded to the nontriggering auditory signal as well as the movement-triggering auditory signal. Twenty-three neurons responded exclusively to the nontriggering auditory signal. These results indicate the extent to which SMA neuronal activity, in contrast to that of the PCM, is related to factors other than the execution of movement.