Gastric acidity of 12 patients with healed duodenal ulcers was continuously monitored over 24 h in order to assess the antisecretory effects of two different administration times of a single daily dose of ranitidine 300 mg. Each patient orally received either (a) placebo at 08.00 h and 22.00 h; (b) ranitidine 300 mg at 08.00 h and placebo at 22.00 h, or (c) placebo at 08.00 h and ranitidine 300 mg at 22.00 h in randomized and double-blind fashion. Each medication was administered on three separate occasions, with intervals of at least 1 week. Both the morning and the bedtime doses of ranitidine were significantly superior (p < 0.001) to placebo in controlling 24-hour gastric acidity, while, in the same period, ranitidine nocte was more effective (p < 0.001) than ranitidine mane. During the night, bedtime ranitidine caused more acid inhibition (p < 0.001) than morning ranitidine, but the opposite (p < 0.01) occurred during the daytime. This study shows that the antisecretory effect of morning ranitidine during the daytime is less consistent than that achieved by bedtime ranitidine during the nocturnal period. As similar rates of duodenal ulcer healing have recently been achieved with morning and conventional nighttime administration of H2 antagonists, it becomes clear that antisecretory drugs can also be beneficial with an acid inhibition which is shorter-lasting than that which was previously thought to be necessary or, alternatively, that also daytime acidity is important in ulcerogenesis.