Eight patients with previous duodenal ulcer in symptomatic remission underwent continuous intraluminal pH monitoring on five separate occasions to compare the effects on 24-h intragastric acidity of placebo, 300 mg ranitidine at night, 150 mg ranitidine twice daily, 40 mg famotidine at night, and 20 mg famotidine twice daily. All H2 blocker treatments were superior to placebo (p congruent to 0), whereas the twice daily doses of both ranitidine and famotidine were significantly better (p congruent to 0 and p = 0.00006, respectively) than the single ones in reducing 24-h intragastric acidity. The higher acid inhibitory effect of the twice daily dose regimens than of the single ones was evident during the daytime, whereas no difference between them was found during the nighttime (from 2200 to 0800 h). These data are at variance with those previously published, and the slight effect of the single nightly doses of H2 blockers on daytime acidity seems to confirm further that the suppression of nocturnal acidity may really be the decisive factor in the success of this dosing schedule in treating duodenal ulcer.