Total extractable N-nitroso compounds, pH, and nitrite levels were measured in, and microorganisms were cultured from, the fasting gastric juice of 140 patients taking the H2-receptor antagonist cimetidine and from 267 subjects, including 50 healthy volunteers, not taking cimetidine. Significantly higher mean N-nitrosamine concentrations and pH levels were demonstrated in the cimetidine-treated patients; N-nitrosamine concentration increased with pH. In 30 patients studied, cimetidine treatment significantly increased gastric pH and N-nitrosamine concentrations, while in 23 patients withdrawal of cimetidine treatment resulted in a significant reduction of pH but not of N-nitrosamine concentrations. The gastric juice nitrite level was often raised and nitrate-reducing bacteria cultured were similar to those associated with other causes of hypochlorhydria. These results demonstrate for the first time a relation between gastric juice N-nitrosamine concentration, pH, and cimetidine treatment, and the findings are discussed in relation to gastric cancer induction.