Tobacco contains specific carcinogenic nitrosamines which are derived from nicotine. These compounds may be among the causative agents for the various cancers (lung, oral cavity, oesophagus, bladder and pancreas) which are associated with tobacco usage. The major tobacco specific nitrosamine is N'-nitrosonornicotine (NNN), which has been detected in both unburned tobacco (0.3-90.6 ppm) and cigarette mainstream smoke (137-238 ng/cig.). Studies with labelled precursors showed that the major source of NNN formed during curing of tobacco was nicotine, rather than nornicotine. The transfer rate of NNN from tobacco to mainstream smoke was 11.3%; about half the NNN present in smoke therefore originated from tobacco, with the remainder being formed during smoking. Model studies of the reaction of nicotine and nitrite showed that, in addition to NNN, two other nitrosamines, 4-(N-methyl-N-nitrosamino)-4-(3-pyridyl)-1-butanal (NNA) and 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) were formed. Analysis of tobacco revealed the presence of NNK in chewing tobacco and snuff (0.6-2.4 ppm). A comparative bioassay of NNN, NNK and NNA in strain A mice indicated that NNK was more tumorigenic than NNN and that NNA was inactive. NNN, which had previously been shown to induce oesophageal and nasal cavity tumours in rats, also was a moderately active carcinogen in the Syrian golden hamster, giving tracheal tumours. A study of the metabolism of cyclic nitrosamines was initiated. Metabolic alpha-hydroxylation of nitrosopyrrolidine, which is thought to be the critical step in activation of this compound, was demonstrated by detection in vitro and in vivo of 2-hydroxytetrahydrofuran, which was the product of decomposition of alpha-hydroxynitrosopyrrolidine. The metabolism of the related cyclic nitrosamine, NNN, is currently under investigation, with emphasis on metabolites resulting from alpha- and beta-hydroxylation. These analytical and metabolic studies are intended to clarify the possible relationship of tobacco-specific nitrosamines and site-specific cancers in tobacco users.