The effects of 1-O-hexyl-2,3,5-trimethylhydroquinone (HTHQ) and other antioxidants on heterocyclic amine (HCA)-induced rat hepatocarcinogenesis were examined in a medium-term liver bioassay. In one study the experimental period was extended for up to 28 weeks to confirm the inhibition of 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1)-induction of glutathione-S-transferase placental form (GST-P) positive foci detected earlier in an 8-week experiment. Six-week-old male F344 rats were given a single i.p. injection of diethylnitrosamine (DEN) (200 mg/kg b.w.), and starting 2 weeks later, groups of 20 animals received a diet containing 0.03% Glu-P-1 together with 0.5% HTHQ, Glu-P-1 alone, HTHQ alone or a basal diet alone for 26 weeks. Three weeks after the DEN injection, animals were subjected to partial hepatectomy. The combined incidence of hepatocellular adenomas and carcinomas in the group fed Glu-P-1 alone was 89%, in contrast to 40% with simultaneous HTHQ treatment, and near the control level of 30% without Glu-P-1 and HTHQ. In the second experiment, to assess the effects of HTHQ on HCAs in combination, to mimic the human situation, after DEN initiation groups of 15 rats received diets containing a 0.0155% HCA mixture (0.003% Glu-P-1, 0.0015% 3-amino-1,4-dimethyl-5-H-pyrido[4,3-b]indole (Trp-P-1), 0.004% 2-amino-3-methyl-9H-pyrido[2,3-b]indole (MeAaC), 0.003% 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 0.004% 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), which are all heptocarcinogens) together with 0.5 or 0.125% HTHQ, HCA alone, 0.5 or 0.125% HTHQ alone, or basal diet alone for 6 weeks. The numbers of GST-P positive foci decreased in a dose-dependent manner to 12.2+/-3.1 and 7.2+/-2.4 by the simultaneous treatment with 0.125 and 0.5% HTHQ, respectively, from a value of 17.6+/-3.6 for the HCA mix alone. In a third experiment, after DEN initiation, groups of 15 rats were placed on diets containing 0.02% MeIQx together with 0.25% HTHQ, 0.05% phenylethyl isothiocyanate (PEITC), 1% green tea catechins (GTC) or a mixture of HTHQ, PEITC and GTC, MeIQx alone, antioxidants alone or in combination, or basal diet alone for 6 weeks. These compounds were previously shown to inhibit HCA-associated GST-P positive foci. The numbers of GST-P positive foci in rats treated with MeIQx together with HTHQ (7.7+/-2.6) or antioxidant mixture (0.4+/-2.8) were significantly lower than with MeIQx alone (12.1+/-3.1), but a clear synergistic effect was not demonstrated. These results confirmed the ability of HTHQ to inhibit hepatocarcinogenesis induced by HCAs.