In 272 nephrectomised rats, 3% hydroxyethyl starch solutions (310 mOsm/l) were compared with colloid-free 2% dextrose solutions as peritoneal dialysis fluid. With regard to effectiveness of dialysis and the rate of ultrafiltration there was no significant difference between the hydroxyethyl starch und dextrose solutions. The oncotic pressure was raised in the peritoneal cavity and the decrease of loss of protein was expected. Nevertheless this could be verified. In all rats dialysed with HES there was an increase of serum concentration of HES and a progressive accumulation of the colloid in different organs (liver, lung, spleen). The lower substituted molecules (40/0,5; 200/0,5) were able to pass the peritoneum more readily than the higher substituted HES molecules (70/0,7; 450/0,7). A further interesting finding was that the elimination rate of HES from the intravascular space is influenced to a greater deal by its degree of substitution than by its molecular weight.