The effects of gomisin A, a lignan compound of Schizandra fruits, on hepatocarcinogenesis induced by 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB) in rats were investigated. Gomisin A inhibited both increases of the number and size of glutathione S-transferase placental form (GST-P)-positive foci, a marker enzyme of preneoplasm, and the population of diploid nuclei, as a proliferative state of hepatocytes, in the liver from rats simultaneously treated with 3'-MeDAB. Gomisin A increased GST activity in the liver, by raising the level of GST 1 and 2 isozymes. 3'-MeDAB increased GST activity and GST-P expression. This high level of GST-P induced by 3'-MeDAB was suppressed by additional treatment with gomisin A. In an experiment on simultaneous treatment, gomisin A increased the biliary excretion of 3'-MeDAB-related aminoazo dyes and decreased the content in the liver of rats fed with 0.06%-3'-MeDAB containing diet. In an experiment on pretreatment with 3'-MeDAB, even though no aminoazo dye was detectable in the liver or bile 2-weeks after cessation of 3'-MeDAB-feeding, gomisin A showed a tendency to reduce the preneoplastic changes of increases in GST-P positive foci and diploid nuclei in the liver. These results suggest that gomisin A inhibits the hepatocarcinogenesis induced by 3'-MeDAB by enhancing the excretion of the carcinogen from the liver and by reversing the normal cytokinesis.