The effects of short-term feeding of mutagen containing, heated deep-frying oils on urinary and faecal mutagenicity, plasma clinical biochemical parameters, peroxidative effects and cell proliferative indices in the gastro-intestinal tract were determined in rats. Repeatedly used frying oils [a saturated fatty acid-rich coconut oil (CO) and a polyunsaturated fatty acid (PUFA)-rich (greater than 60% PUFA) vegetable frying oil (PO)] were administered to groups of seven rats at a level of 10% (by weight) in the diet for 4 wk; control groups were fed equal amounts of the unheated oils. Both heated oils showed direct-acting mutagenicity to Salmonella tester strain TA97; heated PO was also mutagenic to strain TA100. Both heated CO and heated PO contained detectable amounts of thiobarbituric acid-reactive substances (TBA-RS). In heated PO, hydroperoxides of linoleic acid were also present. In groups fed heated oils the mutagenicity of urine and faeces to strain TA97 was not found to be increased in comparison with the control groups. Faecal mutagenicity to strain TA100 was also unaffected by consumption of heated oils. Urinary excretion of TA100 mutagens was significantly increased in rats fed heated PO. Plasma alkaline phosphatase activity was clearly raised in rats fed heated PO, in comparison with rats fed unheated oils or heated CO. In addition, other clinical biochemical plasma parameters showed a tendency to be increased in rats fed heated PO, indicating hepatic and renal cellular toxicity. Urinary and faecal excretion of thiobarbituric acid-reactive substances (TBA-RS) were also slightly, but not significantly, increased in rats fed heated PO. Feeding heated CO to rats did not result in increased plasma enzyme activities and excretion of TBA-RS, nor in increased cell proliferation in gastro-intestinal tissues. Cell proliferation of the oesophageal tissues were slightly, but significantly, increased in rats fed heated PO, in comparison with the group fed unheated PO. Tissues of the glandular stomach and colon/rectum did not show significantly enhanced cell proliferation in the group fed heated PO. The results obtained in this study indicated that consumption of heated oils containing TA100 mutagens and oxidation products of linoleic acid produced indications of cellular damage to liver and kidneys, and increased urine mutagenicity, as well as enhanced cell proliferation in the oesophagus.