The present study was carried out to examine the chemopreventive effects of thiacremonone, which is found in heated garlic, on the formation of the preneoplastic foci and glutathione-dependent detoxifying enzyme activities in rat hepatocarcinogenesis. F344 rats were given an intraperitoneal injection of N-nitrosodiethylamine (DEN) at a dose of 200 mg/kg body weight and were then administered a gavage of thiacremonone 2 weeks after initiating the DEN treatment. All animal were subjected to a 2/3 partial hepatectomy at 3 weeks after DEN initiation. The numbers and the placental glutathione S-transferase positive (GST-$P^+$) foci in the thiacremonone-treated groups decreased significantly compared with those in the DEN alone group. Thiobarbituric acid reactive substances content in the thiacremonone treated group (10 mg/kg) decreased significantly compared with that in the DEN alone. These results suggest that thiacremonone may have potential chemopreventive effects on rat hepatocarcinogenesis induced by DEN.