The effect of experimental acute renal failure on the intestinal permeability measured by differently sized polyethylene glycols (PEG) was studied in rats. The permeability was assessed by analysing the 24-hour urinary recovery of PEG molecules (size 326-1,162 D). Acute renal failure was induced by clamping the left renal artery and right-sided nephrectomy. The overall urinary recovery of PEG was decreased in the uremic rats. However, the relative recovery of larger molecules (range 590-1,162 D) was further reduced in the uremic rats. The results which are compared with a computer simulation by a multicompartment model, suggest that the intestinal permeability especially towards larger PEG is decreased in acute uremic rats.