The in vitro reaction of nitrite with the histamine H2-receptor antagonist ranitidine, in acidified solutions or in human gastric juice, resulted in the formation of genotoxic derivatives, mainly eliciting base-pair substitutions in his-Salmonella typhimurium and trp- Escherichia coli and inducing an increased lethality in DNA repair-deficient bacteria. The mutagenic response was better expressed in the presence of rodent (rat, mouse) and even more of human liver preparations. The patterns of this reaction, e.g., the optimal pH, temperature and time of preincubation, doses of precursor compounds, effect of inhibitors (ascorbic acid), the genotoxic mechanisms and in vitro metabolic trends were investigated and compared with those resulting from nitrosation of cimetidine under the same experimental conditions. Although the reaction proceeded under conditions simulating the gastric environment, an excess of nitrite was needed, in the case of ranitidine, for optimal formation of mutagenic derivatives. Gastric juice samples from fasting individuals treated with ranitidine were devoid of mutagenic activity, and the addition of nitrite to these samples was also without a reproducible effect. Conversely, under the same conditions, most samples from both untreated and ranitidine-treated individuals induced mutations of different genetic specificity (frameshift errors), resulting from nitrosation of physiological components of gastric juice.