Studies of basal and histamine-stimulated gastric acid secretion were performed in 42 controls, in 133 patients with duodenal ulceration, in 57 patients with gastric ulceration, in 13 patients with both duodenal and gastric ulceration, and in 20 patients with gastric carcinoma. All these subjects were Chinese. Statistical analysis of the results showed that all clinical groups differed from the controls in both basal and stimulated secretion. The mean basal acid output of Chinese controls and of patients with duodenal ulcer in most cases did not differ statistically when compared with western series. The basal secretion of Chinese patients with gastric ulceration, however, was statistically higher than in most of the western series. The histamine-stimulated response of Chinese controls and patients with duodenal ulcer was statistically much lower than in all western series with which they were compared. For Chinese patients with gastric ulcer, the stimulated responses were in some instances lower than and in others similar to results obtained in some western series. In contrast to most western reports, the basal and histamine-stimulated secretion in Chinese patients with gastric ulcer was significantly higher than in controls. In the Chinese controls and Chinese patients with peptic ulcers the response after histamine was generally lower than western reports. This was due to the inadequacy of the standard dose of histamine acid phosphate of 0.04 mg/kg body weight in Chinese subjects. A dose of 0.06 mg/kg body weight produced a significantly higher acid output. In the comparatively leaner Chinese subjects, therefore, a dose of histamine acid phosphate of 0.06 mg/kg is required for maximal stimulation of gastric acid secretion. This agrees well with the results of a similar study in Indian subjects.