Platelet function studies were performed on 18 patients with end stage chronic renal failure receiving treatment by long-term peritoneal dialysis. The results of these studies were compared with previously reported data on healthy volunteers, non-dialyzed patients with chronic renal failure and patients treated by hemodialysis. Platelet adhesiveness was impaired and, unlike the other patient groups, there was no relationship between adhesiveness and serum creatinine. Platelet aggregation to both ADP and adrenaline chloride was normal, whereas marked impairment was seen in non-dialyzed patients with severe chronic renal failure and patients receiving hemodialysis. The results suggest that different uremic retention products affect these different phases in the platelet adhesion-aggregation reaction and that the peritoneal membrane is more permeable to that dialyzable factor which inhibits aggregation than conventional hemodialysis membranes. The studies are of interest both in consideration of adequacy of dialysis and in studies into the nature of the platelet defect in uremia.