Of the N-nitroso compounds derived from precursors in gastric juice, only a very small proportion are in the form of volatile N-nitrosamines. However, about 40% of the remainder can be converted by methylation into a form(s) suitable for gas chromatography. In this way, at least 20 individual peaks were detected with the Thermal Energy Analyzer as detector. N-Nitroso compounds have been determined as a group in the fasting gastric juice of normal individuals and patients with conditions including duodenal ulcer, gastric ulcer, atrophic gastritis, pernicious anaemia, gastric carcinoma, dyspepsia with normal endoscopy or subjected to operative procedures, such as vagotomy or partial gastrectomy. Both the levels of N-nitroso compounds and pH values increased significantly with age in normal subjects and in those with pathological conditions. Sex and cigarette smoking had no significant influence. In particular, a positive correlation was found between pH and the level of N-nitroso compounds, with a p value of less than or equal to 10(-6). Their concentration rose from a geometric mean of 0.11 mumol.1(-1) at pH 1.0-1.5, to a value of 1.3 mumol.1(-1) within the pH range 6.5-9.0. Whilst quantitative bacteriology was not carried out, a highly significant relationship was also observed between the concentration of N-nitroso compounds and the availability of nitrate-reducing bacteria. Thus, conditions conducive to gastric cancer are associated with higher levels of compounds responding to group analysis as N-nitrosamines and/or N-nitrosamides.