This paper is concerned with the place of gastrin determination in ulcer surgery. Only the acid-stimulating properties of gastrin are considered. The role of the vagus in the gastrin response to a test meal was investigated in patients with duodenal ulcer. 50 patients were studied before and 3, 6, and 12 months after highly selective vagotomy without (25 patients) and with Wnagensteen-pyloroplasty (25 patients). Basal and stimulated gastrin concentration in serum, as determined by radioimmunoassay, were higher 3 months after the operation and 3h after the test meal gastrin levels were still 50% higher than the basal levels. No difference was found between the patients treated with highly selective vagotomy and the patients treates with HSV and additional pyloroplasty. 6 and 12 months after the operation a moderate decrease of gastrin secretion was observed, but basal as well as stimulated levels were still elevated after 12 months compared to the values before operation. Our results indicate that the pattern of gastrin secretion changes as time passes after highly selective vagotomy with or without additional pyloroplasty.