The short-term effects of hemodialysis on the integrity of the central nervous system (CNS) were examined. Evoked potentials (EP) and performance on five neuropsychologic tests were obtained from six patients undergoing maintenance hemodialysis. Measurements were taken 1, 24, 42, and 66b hours following dialysis. A highly consistent relationship between time since dialysis and EP latency was found. EP latencies tended to decrease during the 1-hour and the 24-hour recording sessions (compared to predialysis recordings), the shortest latencies typically occurring during the 24-hour period, then increasing steadily during the 42-hour and 66-hour intervals. EP amplitude was largest 1 hour after dialysis, then decreased progressively with time since dialysis. Performance on two tests of visual-motor speed and accuracy was best 24 hours after dialysis. The results indicate that there are consistent and measurable changes in the responsivity of the CNS which are related to time since dialysis.