The effect of N'-nitrosoanatabine (NAT) and nicotine on the metabolism of N'-nitrosonornicotine (NNN) and 4-(methyl-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) by cultured rat oral tissue was investigated. The effect of NNN on NNK metabolism and the effect of NNX on NNN metabolism was also determined. NN]K inhibited NNN metabolism more than NNNinhibited NNK metabolism. NAT inhibited the metabolism of NNK but not of NNN. Nicotine, which is present at >500 times the levels of NNN and NNK in smokeless tobacco, inhibited the metabolism of both nitrosamines. Inhibition of1 μM NNN metabolism was greater than that of 1 μM NNK when the concentration of nicotine was 1, 10 or 100 μM. Nicotine at 100 μM inhibited the formation of all metabolites of NNN by 85-92%. These results suggest that NNN and nicotine may be metabolized by a common enzyme.