Standard peritoneal ultrafiltration characteristics in 18 patients undergoing continuous ambulatory peritoneal dialysis (CAPD) were investigated in a total of 21 single dwell studies of 6 h duration with 2 L of 3.86% glucose dialysis fluid. Intraperitoneal dialysate volumes were determined using radioiodinated serum albumin (RISA). Calculations were based on a novel mathematical method in which RISA elimination from the peritoneal cavity was taken into account. The RISA elimination rate, KE, was calculated as 2.1 +/- 0.5 ml/min. The true dialysate volume after 360 min (2957 +/- 196 ml) was on the average 28% lower than the volume (3737 +/- 260 ml) calculated without correction for the elimination of RISA. The mean maximum true volume plus sampling losses was 3255 ml at 240 min corresponding to a mean ultrafiltration of 762 ml between 3 min and 240 min. Our method of peritoneal volume determination proved to be useful for clinical investigations. The present study demonstrates that CAPD patients, without any major ultrafiltration problems, exhibit relatively small interpatient variations in their peritoneal volume over time curve.