We have introduced the term F chronodispersion to denote the scatter or dispersion of the relative latencies of statistically significant numbers of consecutively recorded F waves. Five patients with chronic renal failure and five control subjects were studied. The two groups were matched for age, sex, proximal and distal motor conduction velocity, and latency of fast F waves plotted against distance traveled. We found that the extent of F chronodispersion of the deep peroneal nerve did not exceed 7.5 mscec in control subjects, while it was longer than this value in all uremic patients studied. Measurements of this type may prove to be a useful diagnostic tool for cases in which traditional electrophysiologic measurements are normal.