Changes in food intake, body weight, liver weight, hepatosomatic index and hepatic fatty acid composition were studied in growing male Wistar rats fed a diet containing sunflower oil that had been used repeatedly for frying. During a period of 27 days, the animals were fed semi-synthetic diets containing either 15% used oil with 19.1% polar material (group 2) or 15% unused oils with 5.1% polar material (group 1). Although food intake was similar in both experimental groups, final body weight, body weight gain, food efficiency ratio and protein efficiency ratio were significantly lower in group 2. The increase of the hepatosomatic index in rats of group 2 seems related to a higher ingestion of potential toxic components (dimers and polymers of triglycerides and oxidized triglycerides) by this group. The livers of nine out of the 10 animals studied also showed moderate fibrotic degenerative areas with severe vacuolization, together with regenerative areas containing eosinophilic binuclear hepatocytes. In addition, livers of group 2 animals showed a 12% higher content of arachidonic acid. Data suggest that higher arachidonic acid levels would be needed for regeneration to palliate liver injury. These data suggest that frying sunflower oil is potentially toxic; assessment of risk is therefore necessary for frying fats with a high polar content.