Charge factors and reactive carbonyl solutes may change peritoneal structure and the transport properties of peritoneum. The aim of the present study to analyze the influence of polycationic protamine and glucose degradation products on the diffusive permeability (P) of peritoneal membrane for urea in vitro. Values for diffusion from the interstitial (1) to the mesothelial (M) side of the membrane and in opposite direction are expressed as coefficients of diffusive permeability. Four separate series of experiments were conducted. In the first experiment, transperitoneal transfer of urea (20 mg/dL) in control conditions over 120 minutes was analyzed. In the subsequent three experiments, transport parameters were analyzed before (15 - 60 minutes) and after (75 - 120 minutes) the addition of chemical factors (protamine sulfate 5 mg/dL, glyoxal 10 mg/dL, methylglyoxal 1 mg/dL) on the mesothelial side of the peritoneal membrane. Stability of urea transport was observed in the control series (120 minutes). The mean diffusive permeability coefficients (P = standard error of the mean) were 2.293 +/- 0.211 cm/s and 2.621 +/- 0.457 cm/s (x0.0001) for I --> M and M --> I transfer respectively. Protamine and methylglyoxal did not alter transport, but glyoxal lowered urea M --> I transport by 12% (p < 0.01), with a statistically nonsignificant reduction in opposite direction. Similar modifications are observed in vivo during peritoneal dialysis and may influence the efficiency of renal replacement therapy.