Changes in gastric acid secretion during the 1st year after selective gastric (SGV) and parietal cell (PCV) vagotomy for duodenal ulcer disease were studied. Pentagastrin tests were performed preoperatively and 3 months and 1 year after surgery in 383 SGV and 302 PCV patients. Resting juice pH showed after both operations a trend towards the preoperative distribution from 3 months to 1 year. Basal acid output showed a similar pattern. The initial reduction in pentagastrin-stimulated peak acid output (PAOpg) was most pronounced after SGV, but an increase occurred for both operations during the 1st postoperative year. An exception from this was the minority of patients who had a less than 20% initial reduction. They had a further decrease in their PAOpg. No sex difference and no influence of the duration of symptoms could be demonstrated. The patients with less than 20% initial reduction were younger than the other groups, and this applied for both SGV and PCV. The patterns of change in gastric acid secretion during the first year after vagotomy suggest that biologic factors are active, in addition to the effect of the surgical technique.