Ulnar nerve conduction velocity (NCV) was measured daily using surface electrodes and achieving reproducibility within 2 meters/sec in each patient on a given day. Five normal subjects had a mean NCV of 56.9 meters/sec ±4.0 (SD). The NCV varied daily during a ten-day period and the percent difference between the slowest and most rapid measurements of NCV averaged 17.8% (range 24.6% to 12.4%). Seven uremic patients had a mean NCV of 49.6 meters/sec ± 7.0. The average percent difference in NCV was 16.6% (range 24.9% to 8.4%). The NCV variation was unrelated to other chemical and clinical features, including the electroencephalogram in two patients. It is concluded that because of variation, the NCV is an unreliable tool for following uremic patients unless the changes are marked. It is necessary to determine the degree of daily variation in NCV from one period to another.