In a randomized, double-blind, placebo-controlled, comparative cross-over study, we studied the effect of four H2-receptor antagonists on intragastric 24-hour acidity, nocturnal volume and acid output. Ten healthy male volunteers were administered 300 mg or 150 mg nizatidine, 800 mg cimetidine, 300 mg ranitidine, 40 mg famotidine, or placebo on several days, in each case at 9:000 PM. Nocturnal intragastric H+ concentration (mmol/l) (11:00 PM to 7:00 AM) was significantly reduced by all H2 blockers compared with placebo. We obtained the following inhibition rates: Cimetidine 67%; ranitidine 95%; famotidine 89%; nizatidine 80% (300 mg) and 69% (150 mg). Nocturnal acid (mmol/l) and volume output (ml/h) were also significantly (compared with placebo) inhibited by all four H2-receptor antagonists. Inhibition of nocturnal acid secretion was almost identical on nizatidine 300 mg nocte, ranitidine 300 mg nocte, famotidine 40 mg nocte, and cimetidine 800 mg nocte. Nizatidine 300 mg nocte and 150 mg nocte exclusively reduced acid secretion at night, without an aftereffect into the following day (8:00 AM to 6:00 PM). These results suggest that the clinical efficacy of these H2-receptor antagonists is identical with respect to healing peptic ulcer disease and providing freedom from pain. It is generally accepted today that gastric acid inhibitors used in the treatment of peptic ulcer disease should interfere with daytime gastric acid secretion as little as possible, particularly since the acid protects the stomach from bacteria ingested with the food during the day.