Using ambulatory 24 hour pH monitoring, intragastric acidity was measured in 6 healthy volunteers and 8 patients with duodenal ulcer. According to a latin square design each patient was randomly assigned to receive placebo, 300 mg ranitidine at 19.00 h or 300 mg ranitidine at 22.00 h, on three separate occasions. Validation of the method was achieved by comparing the values indicated by the intragastric electrode and the pH of simultaneously aspirated gastric juice (y = 0.87x + 0.66, r = 0.93). Comparing the area under the curve of intragastric hydrogen ion activity, as well as the percent of time less than pH 5, we found a better inhibition of nocturnal acidity (20.00 h-08.00 h) with 19.00 h ranitidine than with ranitidine administered at 22.00 h (p less than 0.01). By contrast, there was no significant difference in diurnal acidity between both ranitidine regimens and placebo.