Nitrite and hydrogen ion concentrations have been measured in the fasting gastric juice of 69 patients undergoing routine gastrointestinal investigations. There was an inverse relationship between nitrite concentration and hydrogen ion concentration, with a highly significant increase in gastric-juice nitrite in hypochlorhydric subjects. Thiocyanate was also found in all specimens in concentrations likely to increase nitrosamine formation, if nitrosation of amines occurs in the fasting stomach. Neutral gastric juice contains metabolically active bacteria capable both of generating nitrite from nitrate and of catalysing nitrosation. In this way an intragastric environment suitable for the formation of carcinogenic nitrosamines exists in the hypochlorhydric and achlorhydric stomach, providing a possible mechanism for the high incidence of gastric cancer in these subjects.