Relative and absolute refractory periods were determined in sensory fibres of 24 median nerves and in 19 sural nerves of 30 volunteers aged 20–60 years, who had no sign of a neuromuscular disorder. The critical interval of conduction (absolute refractory period) was found to be about 0.7 msec. In median as well as in sural nerve the relative refractory periods were about 5 times the absolute refractory period, measuring the amplitude, and about 3.5 times the absolute refractory period, measuring the latencies. Reasons for the reduction of amplitude and prolongation of latency during the relative refractory period are discussed. Slightly significant differences between the two age groups (20–35 and 40–60 years) have been found with respect to the relative amplitude in median nerves, but not in sural nerves and not with respect to the relative latency in either median or sural n