We have determined the variability of repeated measurements of sensory nerve action potential (SNAP) and compound muscle action potential (CMAP) amplitude and motor and sensory conduction velocity (MCV and SCV) and examined the extent to which limb temperature is responsible for the variability. We made 10 serial measurements of SNAP, CMAP, MCV and SCV in each of 3 nerves in a single normal subject. The coefficients of variation for MCV and SCV ranged from 2.0% to 6.7% and the proportion of the variance due to temperature was 0.3-56%. The coefficients of variation were much greater for serial measurements of compound action potential amplitude. We used the standard deviations for serial measurements in each nerve to calculate the number of subjects required to detect a difference of 1/msec between the means of two sets of measurements with a power of 90%.