A chemical analytic method to determine N' nitrosonornicotine (NNN) and N' nitrosoanabasine (NAB) in tobacco was developed. NNN was found in the unburned tobacco of commercial products at concentrations between 0.3 and 88.6 μg/g. The highest levels were observed in highly fermented snuff (29.1 μg/g) and fine cut chewing tobacco (88.6 μg/g). NAB was not detected (< 0.5 ng/g) in any tobacco examined. Two new tobacco components, N' carbomethoxynornicotine and N' carbomethoxyanabasine, were found. Possible origins of NNN in tobacco were discussed, especially in relation to concentrations of nitrite, nitrate, and alkaloids, and in relation to pH and curing. Biologic implications, including the possible function of NNN (the first organic carcinogen isolated from unburned tobacco) as a causative factor in cancer of the oral cavity in tobacco chewers and betel quid chewers, were also discussed.