Model studies on lung cancer in animals have shown that adenoma and adenocarcinoma are specifically elicited by nicotine-derived tobacco-specific N-nitrosamines (TSNA), such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). Lowering or entirely eliminating TSNA in tobacco and in cigarette smoke might, in theory, reduce a large portion of the cancer-causing potential of these products. Recent technologic advances have led to the production of a tobacco that is essentially free of TSNA. To examine the effects of smoking cigarettes produced from such modified tobacco on carbon monoxide (CO) burden, and nicotine uptake and metabolism, and to verify whether uptake and metabolism of NNK is indeed reduced, we determined the CO levels in expired air and analyzed urine specimens from 11 smokers who initially smoked full-flavored American blended cigarettes (FTC nicotine yield: >=1 mg/cigarette) and then switched for 9 days to an experimental cigarette CigRxTM containing only Virginia tobacco with minimal or no TSNA and no added flavorings, but the same level of nicotine as their usual brands. Switching to the CigRxTM cigarette lowered CO levels by 40% and reduced urinary levels of the major NNK metabolites, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and its glucuronic acid conjugate, by 50-70%. Cotinine, the biomarker of nicotine exposure, remained at the levels determined when subjects were smoking their usual brands. Urinary NNAL was not completely eliminated after switching to the TSNA-free cigarette, but this finding is in line with others showing NNAL persistence well over 120 days after complete cessation of smoking. In summary, smoking cigarettes produced from TSNA-free tobacco significantly reduced the body burden of the lung carcinogen NNK without increasing the exposure to CO.