The gastric secretion of hydrochloric acid, total proteolytic activity, and especially pepsin were studied before and 6 months after selective proximal vagotomy in 15 patients suffering from recurrent duodenal ulcer, and compared with that of ten sound volunteers. The patients with duodenal ulcer showed a significantly increased secretion of acid and proteolytic enzymes. After vagotomy, the basal acid output was reduced by 70%, the secretion of the total proteolytic activity by 54%, and that of pepsin by 45%. The concentration of the total proteolytic activity and of pepsin in the gastric juice was increased after vagotomy. Although the total basal proteolytic activity after vagotomy was still increased by 15% as compared with that in the volunteers, due to the drastically reduced basal acid output the actual intragastric basal proteolytic activity was expected to be reduced after vagotomy.