1. Primary somatosensory cortical (SI) neurons exhibit characteristic activity before the initiation of movements. This premovement activity (PMA) may result from centrally generated as well as from peripheral inputs. We examined PMA for 55 SI neurons (10, 13, 28, and 4 in areas 3a, 3b, 1, and 2, respectively) with activity that was entrained to vibrotactile stimulation (i.e., was temporally correlated with the stimulus). We sought to determine whether the temporal characteristics of vibration-entrained discharges would change throughout the reaction time period, and, if they did, whether these changes might be accounted for by central inputs. 2. Monkeys made wrist flexions and extensions in response to sinusoidal vibration (27, 57, or 127 Hz) of their palms. Vibration remained on until the animal moved at least 5° from the initial hold position. Mean firing rate (MFR), a measure of the level of activity, was derived from the number of spikes per vibratory cycle. The correlation between the vibration and the neuronal firing was described by the mean phase (MP) of the vibratory cycle at which spikes occurred. The degree of entrainment was quantified as synchronicity (Synch), a statistical parameter that could change from 0 for no entrainment to 1 for responses at a constant phase. 3. Premovement MFR increases (activation) and decreases (suppression) were observed. Moreover, two changes in MFR often were observed for the same neuron (2-event PMA). Many MFR shifts, especially the first in the two-event PMA, preceded electromyographic (EMG) onset. The pre-EMG MFR shifts more often had the same sign both for flexion and extension movements rather than having opposite signs. However, with equal frequency, post-EMG PMA events had the same or opposite sign for different movement directions. We suggest that the pre-EMG PMA has an origin different from movement- related peripheral reafference. 4. Premovement activation was accompanied by shifts of MP corresponding to earlier responses to the ongoing vibratory stimulus and by decreases of response Synch. Premovement suppression was not associated with consistent shifts of MP and Synch. We suggest that during premovement activation, asynchronous (uncorrelated with vibration) signals are integrated with the vibratory input. These asynchronous signals may make neurons more likely to discharge and to do so earlier with respect to the vibratory stimulus. The asynchronous component may also disrupt the vibration-entrained activity pattern. 5. From these data we conclude that the activity of SI neurons that most faithfully represent the sensory periphery is modulated before voluntary movements. We suggest that inputs of central origin may contribute to this premovement modulation. Presumably, the role of the central inputs may be to prepare SI for changes in sensory activity that result from voluntary movement. Premovement asynchronous signals, both of central and peripheral origin, disrupt the fidelity of coding of the vibratory stimulus by SI neurons. This deterioration of the quality of representation of sensory inputs by SI neurons may be related to the phenomena of premovement elevation of the tactile threshold of perception and of premovement decrease of somatosensory-evoked potentials.