A simple method has been devised to measure the refractory period of motor fibers in man. With paired shocks of just maximal intensity, the second response of the pair was first elicited at an interstimulus interval of 1.00+/-0.20 msec (mean +/- SD in 24 ulnar nerves). With longer intervals there was progressive return of excitability until it recovered completely at 2.88+/-0.72 msec. When paired shocks of 50% supramaximal intensity were used, return of the second response was faster, beginning at 0.77+/-0.18 msec and achieving full recovery at 2.03+/-0.57 msec. The inpulse was conducted at a slower speed than normal, if transmitted at all, during the relative refractory period.