Mental or neurobehavioral manifestations of uremia usually are the earliest and the most responsive to dialysis and to renal transplantation. An assay system relevant to uremia is capable of identifying toxic factors by quantitive neurobehavioral end points. In this system chemical composition of the body fluids is maintained by peritoneal dialysis. Mental performance by appropriately trained monkeys deteriorated following ureteral ligation when blood urea nitrogen (BUN) reached levels of 50 to 200 mg/100 ml, and was restored toward normal by peritoneal dialysis, even though BUN concentration was maintained at a high level by adding urea to the dialysate. Distribution of power in simultaneously recorded electroencephalograms favored slower frequencies as BUN level rose, with reversal following dialysis.