The antisecretory efficacy of a single bedtime dose of famotidine, a new potent H2-receptor antagonist, was evaluated by means of continuous 24-hour intragastric pH monitoring. Of 20 patients with duodenal ulcers, ten randomly received famotidine 40 mg at 10 PM and ten were monitored without medication for control. Famotidine regimen led to a remarkable reduction of gastric acidity in patients who were treated for duodenal ulcer and the drug-induced pH levels were significantly different (P less than .0001) from those of untreated controls. The antisecretory action lasted for 12 hours, which comprised the nocturnal period, whereas no important difference was found between the two groups for the most part of the daytime. The drug was able to keep intragastric pH above 4 units during almost 50% of the whole 24-hour period. These results confirm that famotidine is a powerful and long-acting H2 blocker that relieves gastric acidity during the night and morning hours when administered as a single bedtime dose of 40 mg.