We evaluated the effect of 6 days of intraperitoneal saline infusion on peritoneal peroxidation and permeability in rats. Peroxidation of the peritoneum as measured by malondialdehyde concentration in the omentum was increased and there was a concomitant augmented membrane permeability to glucose and resulted in a loss of ultrafiltration. In vitro experiments with mesothelial cells showed that glycosaminoglycan chondroitin sulphate appears to act as a scavenger of free radicals and so protects the mesothelial cells against injury. Thus, in rats, supplementation of the infused saline with chondroitin sulphate reduces peroxidation of the peritoneum and prevents loss of ultrafiltration during peritoneal dialysis. These results suggest that chondroitin sulphate may be effective in preventing the deterioration of peritoneal permeability during chronic peritoneal dialysis. This beneficial effect probably derives from the scavenging of free radicals by chondroitin sulphate.