We have studied the effect of intraperitoneal nitroprusside on small (urea) and middle (vitamin B12) molecule dialysis clearances in patients maintained on chronic peritoneal dialysis. Clearances were measured in six patients during the addition of 6 mg of nitroprusside to three and seven consecutive, hourly two-liter exchanges during the course of a routine dialysis treatment. Results indicate that clearances of urea and B12 both increased about 35 percent with the addition of the vasodilator for three exchanges. Clearances immediately returned to baseline values when administration of the vasodilator was discontinued. Addition of the drug to seven consecutive exchanges resulted in a sustained 35 percent increment in clearances. We conclude that addition of 6 mg of nitroprusside to peritoneal dialysis solution can result in a significant increment in both small and middle molecule clearances. Maintenance of the higher clearances requires sustained administration of the drug, which can be tolerated for at least six exchanges with no adverse side effects.