Gastric acid secretion was measured in six healthy volunteers following the intravenous administration (over 30 min.) of oxmetidine 200 mg, 400 mg and 800 mg in a randomized double blind trial for 12 hours. Gastric aspirates were fractionated into hourly aliquots and pH, volume, gastric acidity and gastric acid output were determined. Total gastric acid output over 12 hours was significantly reduced from 24.4 mmol/12 h (median) after placebo to 13.5 mmol/12 h, 11.8 mmol/12 h and 7.7 mmol/12 h after oxmetidine 200 mg, 400 mg and 800 mg respectively. An inhibition of at least 90% was achieved with all doses of oxmetidine and this lasted dose-dependently for 4 to 6 hours. A rise in pH to greater than 5 occurred during the 2nd or 3rd hour after dosing which lasted for 2-5 hours depending on the dose administered. Mean hourly pH was dose-dependently significantly higher for 4-6 hours following oxmetidine treatment than after placebo. A significant reduction in gastric acidity after oxmetidine infusion was also observed while the reduction in volume output calculated for 12 hours was not statistically significant. No significant rises in serum gastrin levels were observed with the oxmetidine doses used. Three out of the six subjects tested showed an increase in serum prolactin levels in response to the highest dose of oxmetidine but this was not statistically significant. The results of the present study have shown that the administration of 400 mg oxmetidine did not cause a longer pH elevation greater than 5 than 200 mg, while with 800 mg side effects were observed in one of the six subjects studied.(ABSTRACT TRUNCATED AT 250 WORDS)