The effect of intragastric and intraduodenal 16,16-dimethyl prostaglandin E2 (dm PGE2) on meal-stimulated gastric acid secretion and gastrin release was studied in patients with inactive duodenal ulcer. Compared to placebo, doses of 0.75, 1.00, 1.33, and 1.77 mug per kg of dm PGE2 instilled into the stomach inhibited meal-stimulated gastric acid secretion by 61 to 94% (P less than 0.01). The 1.00, 1.33, and 1.77 mug per kg doses inhibited acid secretion significantly (P less than 0.05) more than an optimal dose of propantheline bromide. Intragastric dm PGE2 (1 mug per kg) was significantly (P less than 0.05) more effective than intraduodenal dm PGE2 (1 mug per kg) in inhibiting both gastric acid secretion and gastrin release. After 1.33 and 1.77 mug per kg, some patients experienced abdominal cramps, or diarrhea, or both, but at doses of 1.00 mug per kg or less no apparent untoward side effects were observed. It is concluded that 16,16-dm PGE2 significantly inhibits meal-stimulated gastric acid secretion and gastrin release, and may be of therapeutic value in patients with peptic ulcer provided it is free of untoward side-effects with chronic administration.