The firing of 97 soleus motor units (MUs) and 52 triceps brachii MUs of eight volunteers has been studied in the tonic vibration reflex. The vibration frequency was varied from 30 to 150 Hz. Interspike interval histograms of single MUs as well as cross-correlograms between their spikes and vibration mechanograms were obtained to reveal the correlation between MU discharges and vibration stimuli. In all of the MUs studied, both methods revealed the correlation. More detailed study of the discharge pattern of soleus MUs revealed the dependence of the correlation on the vibration frequency: the highest degree of correlation was observed at low vibration frequencies (30-50 Hz); the correlation gradually decreased with the increase in vibration frequency and disappeared with frequencies higher than 85 Hz. However, the correlation was observed when high frequency vibration was applied to a voluntarily contracted muscle. The muscle contraction also removed the vibration-induced depression of the soleus H reflex. At low frequency vibration, the correlation may be caused by the group II afferent EPSPs. A voluntary muscle contraction seems to be accompanied by removal of the vibration-induced presynaptic inhibition of the monosynaptic input to motoneurons from Ia afferents that leads to the correlation at high vibration frequencies.