The purpose of the present series of tests was to measure and compare the effects of ingestion of gelatin capsules containing 15(R)-15-methyl PGE2 (PG) and/or an anticholinergic drug (methscopolamine bromide, Pamine) on meal-induced gastric acid secretion and serum gastrin level. Eleven duodenal ulcer patients were stimulated by a 5% peptone meal. Acid secretion was determined by the intragastric titration technique, and serum gastrin was measured by radioimmunoassay. The tests were randomized and double-blind. PG alone given 30 min before a test meal at a dose of 50 micrograms or 100 micrograms produced no side effects and inhibited meal-stimulated acid secretion by about 43% and 55%, respectively. Gastric acid inhibition after a single dose of PG was most pronounced in the first hour of a test meal and was accompanied by almost complete suppression of the meal-induced serum gastrin level. Pamine alone in a dose of 2.5 mg reduced gastric acid response to a meal by about 29% but caused a further rise of postprandial serum gastrin level over control values. The combination of PG, 50 micrograms, and Pamine, 2.5 mg, did not result in significantly greater acid inhibition (about 48%) than when either compound was given alone. When the higher dose of PG (100 micrograms) was given together with Pamine (2.5 mg), the degree of inhibition produced by PG alone was not changed. It is concluded that PG given orally in capsules is a potent inhibitor of gastric acid and serum gastrin response to a meal and that this effect may be of potential value in the treatment of peptic ulcer disease.