SUMMARYIntragastric pH was measured continuously from 1800 to 1200 hours the following day in 22 duodenal ulcer patients and in eight gastric ulcer patients, all of whom had been admitted as emergencies with acute upper gastrointestinal haemorrhage. The effects of intravenous cimetidine or ranitidine were compared with no treatment. In patients with duodenal ulcer, median intragastric pH was 1.8 (range 1.0–4.9) in the group receiving no treatment. In the cimetidine group (400 mg, 6‐hourly,n= 8) median pH was 4.7 (range 1.5–7.7) and after ranitidine (50 mg, 6‐hourly,n= 10) it was 3.8 (range 1.2–7.8). The pH remained above 4.0 for 67% of the recording time with cimetidine, 47% with ranitidine and for only 3% with placebo. Intragastric pH in gastric ulcer patients without treatment was higher (median 3.4, range 1.0–6.9) than in duodenal ulcer patients with treatment. Both H2antagonists raised intragastric pH in patients with gastric ulcer and maintained a gastric pH of>4.0 for at least 50% of the time. Presently recommended i.v. doses of cimetidine and ranitidine do not consistently maintain gastric pH above 4.0 for long periods in patients with peptic ulc