Twenty-four hour intragastric acidity and nocturnal acid secretion were measured in 10 males with duodenal ulcer in four separate 24 hour studies, during which the subjects ate normal meals, had unrestricted physical activity, and consumed their customary quantities of tobacco. The medication consisted of either placebo, cimetidine 200 mg tds and 400 mg at night, or ranitidine 150 mg bd, or 200 mg bd. Ranitidine 150 mg bd decreased mean 24 hour hydrogen ion activity from 41.8 mmol/l to 13.1 mmol/l (-69%, P less than 0.001) and nocturnal acid output from 6.1 mmol/h to 0.6 mmol/h (-90%, P less than 0.01). This degree of inhibition was significantly greater than that due to cimetidine (P less than 0.001 for 24 hours acidity, less than 0.05 for night time acid output). Plasma concentrations of ranitidine were greater than the IC50 for more than eight hours after the 150 mg dose. Ranitidine 200 mg conferred no additional advantage. Ranitidine 150 mg bd should be tested in therapeutic trials.