A comparison was made of the antisecretory activity of orally administered nizatidine and ranitidine by measuring intragastric pH under basal conditions and during and after pentagastrin stimulation. Intragastric pH values were measured with a bipolar glass electrode in 10 patients with healed duodenal ulcers treated with nizatidine or ranitidine according to a randomized single-blind design. The antisecretory activity of the two drugs was similar during the 4 h of monitoring following drug administration. Nizatidine, however, showed a more rapid inhibitory action than ranitidine, producing a significantly greater increase in pH with respect to basal values during pentagastrin infusion. In the period following infusion the pH values observed with ranitidine were higher than with nizatidine, but not significantly so. Under these experimental conditions, therefore, the antisecretory activity of nizatidine was shown to be more rapid than that of ranitidine and equally effective.