The effects of peritoneal albumin loss and the consequences of heterogeneous peritoneal solute transport on serum albumin levels were investigated in 25 adult patients on standard continuous ambulatory peritoneal dialysis (0-58 months). The patients were divided into three groups according to their albumin concentrations (g/l/l.73 m2) in 8-hour overnight effluents: group 1 ( < 0.6, n = 5), group 2 (0.6-1.1; n = 14), and group 3 ( > 1.1;n= 6). Significant differences (mean +/- SD) were observed in serum albumin levels (4.4 +/- 0.2, 3.7 +/- 0.3, and 3.1 +/- 0.5 g/dl, respectively) and in net ultrafiltration (0.37 +/- 0.13, 0.19 +/- 0.21, and -0.06 +/- 0.20 liters/8 h/1.73m2, respectively). The serum albumin levels were strongly correlated with 8-hour peritoneal mass transfer, clearance of albumin, 8-hour effluent concentrations of protein and glucose, and ultrafiltration rate. Moreover, the serum albumin levels showed significant negative correlations with dialysate-to-serum ratios of small solutes (urea, creatinine, and uric acid) and macromolecules (IgG, IgA, and IgM) estimated from 8-, 4-, and 1-hour dwell times. In addition, an overnight dialysate glucose-to-protein ratio < 1.0 was highly predictive of low serum albumin levels ( < or = 3.5 g/dl) and poor ultrafiltration. From the results of this study we conclude that peritoneal loss of albumin as well as peritoneal transport of other solutes of wide size ('permeability') contribute to the low serum albumin levels during continuous ambulatory peritoneal dialysis, especially in patients with a high peritoneal permeability.