The clinical implications of N-nitrosamines (NAs) were studied by analyzing their concentration in the gastric juice of 72 healthy subjects and 279 patients with gastric ulcer before and during treatment with histamine H2-receptor antagonists. NAs were measured by combined gas chromatography and thermal energy analyzer. The detection ratios of N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) in the patients were 35.3% and 34.6%, respectively, which were significantly higher than the corresponding values in healthy subjects (19.4% and 16.7%, P less than 0.01). Analysis among the patients showed that this trend was mainly due to higher values in patients who were given histamine H2-receptor antagonists, as their detection ratios increased to 40.2% (NDMA) and 39.9% (NDEA). Patients without histamine H2-receptor antagonists showed moderate increases of detection ratios (NDMA; 24.2% NDEA; 22.6%) compared with healthy controls. The differences in these values between those receiving and not receiving histamine H2-receptor antagonists were statistically significant (P less than 0.01). The maximum concentrations of NDMA and NDEA were 7.9 and 9.8 ng/ml in patients, and 1.2 and 1.3 ng/ml in healthy subjects (the difference between the 2 groups P less than 0.02). These results indicated that patients with gastric ulcer had higher detection ratios and concentrations of NDMA and NDEA in gastric juice and that, while significant increases occurred during treatment with histamine H2-receptor antagonists, the extent of increase was below toxic or experimental carcinogenic levels.