1. We observed in a previous study on the human foot dorsiflexor muscles that the fatigue-induced decline in motor output during sustained maximal voluntary contractions (MVCs) was temporarily counteracted during the initial phase of superimposed high-frequency (150 Hz) muscle vibration, whereas prolonged muscle vibration seemed to accentuate the fatigue-induced decline in gross EMG activity and motor unit firing rates. A more extensive investigation of this late effect of muscle vibration on MVCs was performed in the present study. 2. Prolonged periods of superimposed muscle vibration caused a reduction of EMG activity, motor unit firing rates and contraction force in both intermittent and sustained MVCs. This vibration-induced effect had the following main characteristics: (i) it developed slowly during the course of about 1 min of sustained vibration and subsided within 10-20 s after the end of vibration; (ii) it was much more pronounced in some subjects than in others (not age-dependent) and it was accentuated by preceding muscle exercise; (iii) it affected primarily the subject's ability to generate and/or maintain high firing rates in high-threshold motor units. 3. Since the effect developed while vibration at the same time exerted a tonic excitatory influence on the alpha-motoneurone pool (as evidenced by the presence of a tonic vibration reflex) it is argued that the vibration-induced suppression of motor output in MVCs probably does not depend on alpha-motoneurone inhibition, but on a reduced accessibility of these neurones to the voluntary commands. It is suggested that contributing mechanisms might be vibration-induced presynaptic inhibition and/or 'transmitter depletion' in the group Ia excitatory pathways which constitute the afferent link of the gamma-loop.