(+)-Catechin is a plant flavonoid which decreases the mutagenicity of several mutagens and carcinogens. In this study, we have investigated how (+)-catechin could inhibit the metabolism and DNA damage induced by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific carcinogen. Addition of 5 to 1000 microM (+)-catechin to rat hepatocytes cultured with 4.5 mM NNK caused a concentration-dependent reduction of alpha-carbon hydroxylation which is the activation pathway of NNK. Under the same conditions, (+)-catechin had a less significant effect on pyridine N-oxidation, which is a deactivation pathway. Reduction of the carbonyl group of NNK was not inhibited by (+)-catechin. We had previously shown that NNK induced single-strand breaks (SSBs) in primary culture of hepatocytes. In this study, we observed that 1.0 mM (+)-catechin inhibited the DNA SSBs induced by 1 mM NNK by 31%. With 1 mM N-nitrosodimethylamine, the inhibition of DNA SSBs was 30%. We concluded that (+)-catechin selectively inhibits the enzymes involved in the activation of NNK. Rats were gavaged with (+)-catechin (1.5 mmol/kg), injected s.c. 1 h later with NNK (0.39 mmol/kg) and killed 4 h after NNK treatment. (+)-Catechin significantly reduced DNA SSBs induced by NNK. Rats were injected s.c. with 0.39 mmol/kg NNK. (+)-Catechin reduced the methylation of liver DNA at the O6-guanine and N-7 guanine sites by 28 and 34% respectively. These results demonstrate that (+)-catechin inhibits the formation of DNA-damaging intermediates by selectively impairing the enzymatic activation of NNK. They suggest that (+)-catechin could be an effective preventive agent against NNK hepatocarcinogenicity.