Tobacco-specific N-nitrosamines (TSNA) are the most abundant carcinogens identified in tobacco and its smoke. Reducing their levels in tobacco products and especially in cigarette smoke is, therefore, a primary goal towards minimizing the carcinogenic burden of the tobacco consumer. This study delineates the mechanisms of formation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), the most powerful of the carcinogenic TSNA during cigarette smoking. It demonstrates, by means of radiolabeled tracer compounds that 6.9-11.0% of the NNK formed in tobacco during the curing process transfers into the mainstream smoke. This constitutes 26-37% of the NNK present in the smoke. Addition of [methyl-14C]-nicotine to cigarettes, prior to smoking, led to the finding that 0.001% of nicotine in the cigarette column appears in the smoke as NNK. Thus, 63-74% of NNK in smoke is formed during smoking. NNK yield in the smoke was independent of nitrate content of the tobacco. These data serve to devise methods of reducing TSNA in smoke.