We continuously monitored 24-h intragastric pH in eight ulcer patients--who received orally at 10 PM in double-blind, randomized fashion either placebo, ranitidine 150 mg and 300 mg, or famotidine 20 mg and 40 mg, on five separate occasions--in order to determine whether half the commonly used bedtime doses of the H2 antagonists would suppress overnight acidity to the same extent as the large doses. Our results show that, during the nocturnal period (from 11 PM to 8 AM), significantly higher pH values were obtained with the large doses than with the half doses of both ranitidine (p = 0.00005) and famotidine (p = 0.00004). However, hydrogen ion activity was virtually nil with each H2 blocker dose regimen, and the percent inhibition of acidity over placebo was 100% for all of them (p = approximately equal to 0). Further more, with regard to the nocturnal period elapsed in min above 5.0 pH units, there was no significant difference between the two ranitidine doses (p = 0.39) and the two famotidine doses (p = 0.81). Therefore, the two dosing schedules of each H2 antagonist increased intragastric pH differently, but both the half and the standard large regimens produced similar overnight virtual anacidity. It is suggested that ranitidine and famotidine should be evaluated in the acute treatment of duodenal ulcer, using single bedtime doses half those commonly employed.