Objective To determine whether intraperitoneal (IP) administration of amino acids improves nitrogen balance during continuous ambulatory peritoneal dialysis (CAPD). Design Nitrogen balance studies of CAPD patients comparing periods of peritoneal dialysis using dialysate with nitrogen added from a commercial amino acid solution, versus standard dialysate. Setting Clinical research center in a tertiary facility. Patients Patients on CAPD who volunteered. Interventions One 4-day balance period was carried out with conventional dialysate and one 4-day period with conventional dialysate to which amino acids were added. The study periods were separated by a 3-day washout period. Main Outcome Measures Nitrogen accumulation or loss during each period. Results The mean (SD) daily control period balances (grams per day) were 0.2, 1.9, and -3.4; during the amino acid period the balances were 5.4, 2.7, -0.7. The mean daily control effluent dialysate volumes (milliliters) were 11 350±443, 10 052±449, and 10 165±154; during the amino acid period the volumes were 10345±50, 12325±1021, and 10 220±655. Conclusions The IP addition of amino acids diminishes the amount of nitrogen lost from dialysate while maintaining effluent dialysate volume. Nitrogen balances improved in al13 patients during the period with IP amino acids, compared to control.