We evaluated two equivalent regimens of cimetidine to raise intragastric pH over a 24-hr period, using a glass pH electrode in five patients with duodenal ulcer. Each patient received the following drug in a randomized fashion: cimetidine 200 mg after each meal and at bedtime (200 mg qid), cimetidine 400 mg after breakfast and at bedtime (400 mg bid), or placebo tablets (control). Mean intragastric acidity for 24 hr was suppressed by 44.0% in 200 mg qid, and by 73.7% in 400 mg bid of control. In particular, nocturnal gastric acidity was suppressed by 63.1% and 91.3% in 200 mg qid and 400 mg bid, respectively. Thus, 400 mg bid was more effective in lowering gastric acidity than 200 mg qid, although the total daily dose of cimetidine in the two regimens was the same.