Determination was made of in situ gastric pH during early morning in fasting state (basal pH) by using wired glass electrodes, and results obtained were analyzed. Analysis of the pattern of variation in intragastric pH at night revealed no substantial difference between normal subjects and patients with peptic ulcer but a tendency to be lower in the latter group. It was also shown that sleeping waves appeared in the electroencephalogram in association with the increase in intragastric pH during sleep at night. The basal pH value was 5.4 +/- 2.1 in patients with gastric cancer, 3.0 +/- 2.2 in those with gastritis, 2.4 +/- 1.9 in those with gastric ulcer, 1.7 +/- 0.2 in normal subjects and 1.3 +/- 0.6 in patients with duodenal ulcer. In gastric ulcer patients more anal site of ulcer lesion was associated with lower mean age of the patients and higher incidence of intestinal metaplastic gastritis of the antral or non-metaplastic type. In patients who underwent partial gastrectomy for peptic ulcer, the pH value in the remnant stomach tended to become higher with the lapse of time in all cases, being constant at about 3 months postoperatively. The decrease in gastric acidity at 12 months after operation was incomplete in patients who underwent emergency gastrectomy for perforated duodenal ulcer but satisfactory in those who underwent selective vatotomy and anterectomy as elective operations.