In sixteen patients with chronic duodenal ulcer, the effect of oral and intravenous propantheline bromide on gastric acid secretion was compared with that of surgical truncal vagotomy. Basal acid output was reduced from control values by both oral propantheline (81.03%) and surgical truncal vagotomy (79.09%). Acid output in response to histamine also was reduced from control values by both oral propantheline and surgical truncal vagotomy, but to a lesser degree (35.23% and 39.67% respectively). The correlation between values obtained after oral propantheline and after surgical vagotomy was high for both basal and histamine-stimulated secretion (r=0.94 and 0.84 respectively). Intravenous propantheline reduced both basal (44.86%) and histamine-stimulated secretion (21.80%), but to a lesser degree than did either oral propantheline or surgical truncal vagotomy. The correlation with values after surgical vagotomy was low (r=0.36 basal and 0.43 posthistamine). Our data show that “oral medical vagotomy” with propantheline bromide may be useful in predicting the effect of surgical vagotomy on gastric acid secretion.