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1. The electrical activity of single motor units has been recorded from the first dorsal interosseus muscle of normal human subjects during voluntary, isometric contractions, together with the force generated by the muscle.

2. By averaging the force correlated with the impulses from a single motor unit, the contraction time and twitch tension generated by that motor unit could be measured. When the rate of discharge was limited, either voluntarily or by automatic selection of intervals for analysis, the time for the tension to decline to half its maximum value (half-relaxation time) could also be measured for some motor units.

3. Under our experimental conditions the trains of impulses from different motor units in most subjects were generated quite independently as tested by (a) measuring the correlation between activity in single units and that in the whole muscle as recorded by the surface electromyogram (e.m.g.), (b) measuring the cross-correlations between pairs of single units and (c) comparing the tension generated by stimulating single motor units with the average tension correlated in time with voluntary activity of single units in the same location.

4. In one normal subject evidence of synchronization between separate motor units was obtained. Cross-correlation studies suggested that the cause of the synchronization was the presence of substantial common excitation received by the various motor units in the muscle.

5. The frequency response for the contractions of single motor units was well fitted by that for a linear, second-order system with nearly critical damping. However, when stimulation of a few motor units was superimposed on a voluntary contraction, underdamped (oscillatory) responses were seen which were probably of reflex origin.

6. The significance of these results in relation to the normal postural tremor in hand muscles is discussed.