Accurate residual volume (RV) measurements are needed in studies on fluid kinetics during CAPD. In this study 10 stable CAPD patients were examined twice within 1 week. On both occasions RV after drainage was calculated by the indicator dilution method. Exogenous (dextran 70, inulin) and endogenous (albumin, IgG, urea, creatinine) solutes were used as markers. RV calculated with endogenous solutes (mean +/- SD) were significantly higher than those calculated with dextran (232 +/- 77 mL) and inulin (223 +/- 73): albumin (389 +/- 123), IgG (497 +/- 180), urea (465 +/- 129) and creatinine (429 +/- 109). The relationship between RV calculated with exogenous solutes was much better than between those calculated with endogenous solutes: dextran vs inulin (r = 0.91), albumin vs IgG (r = 0.69) and urea vs creatinine (r = 0.63). Since mass transport of endogenous solutes during rinsing time exceeds mass transport of dextran and inulin, RV was also calculated after corrections had been made for diffusive mass transport of endogenous solutes during the rinsing procedure. After this correction only albumin was similar to exogenous solutes (244 +/- 111 mL) and had an acceptable confidence interval when compared to dextran. No correlation was found between RV on the first and second day, suggesting large intra-individual variability. We conclude that RV should be calculated with dextran or inulin. When no exogenous solutes are used, albumin is the best alternative. However, only rough estimations are obtained when no correction for diffusion is applied.