Blood flow in the human stomach was measured during operation with a 85Krypton washout method which made simultaneous determinations of total blood flow and intramural flow distribution possible. The antrum and the corpus of the stomach could be investigated separately. Eleven patients with duodenal ulcer disease were studied during pentagastrin infusion and after the addition of cimetidine, 3 mg/kg bw, to evaluate the effect of the drug on augmented gastric blood flow. Eight recordings were made over the corpus of the stomach and three recordings over the antrum. Cimetidine caused a 66 +/- 5% decrease (mean +/- SE; range 56-86) in acid secretion and a 62 +/- 5% decrease (range 44-91) in the corpus mucosal blood flow within 15 minutes. Changes were only seen in the acid secreting part of the stomach while the antral circulation remained unaltered. It is concluded that the decrease in pentagastrin induced vasodilatation in the stomach seen after giving cimetidine was secondary to an inhibition of acid secretion.