Differences have been reported in the decline of residual renal function in patients on continuous ambulatory peritoneal dialysis (CAPD) and hemodialysis (HD), but it is unknown whether the urinary handling of water and solutes is similar in these patient groups. Ten CAPD patients with residual renal function were investigated during a clearance period (CP) of 24 hours, and 11 HD patients were investigated during one interdialytic interval of three days. In CAPD patients the urinary volume excretion was 0.65 +/- 0.31 mL/min (mean +/- SD), and the inulin clearance was 3.85 +/- 2.82 mL/min. A negative correlation was found between the peritoneal net ultrafiltration rate and both the urinary volume excretion rate (r = -0.80, p < 0.01) and the fractional sodium clearance (r = -0.69, p < 0.05). In HD patients the urinary volume excretion increased from 0.36 +/- 0.36 mL/min during the initial eight hours of CP (HD-A) to 0.64 +/- 0.29 mL/min during the last ten hours of CP (HD-D, p < 0.05), and the inulin clearance increased from 1.9 +/- 1.3 (HD-A) to 2.9 +/- 1.1 (HD-D, p < 0.005). The fractional sodium clearance increased from 8.5 +/- 5.7% (HD-A) to 14.4 +/- 9.0% (HD-D, p < 0.05). It can be concluded that the fractional excretion of volume and fractional clearance of solutes were similar in patients treated with CAPD and hemodialysis. The most important regulating factor seems to be the volume status influenced by volume removal by peritoneal net ultrafiltration in CAPD patients, and volume expansion during the interdialytic interval in hemodialysis patients.