Pyloric sphincter pressure was assessed with water-perfused polyvinyl tubes. Smoking one cigarette significantly decreased the basal pyloric pressure, whereas 10 mg of metoclopramide as an intravenous bolus increased the pyloric pressure in normal subjects and in patients with gastric ulcer with low basal pressure. Duodenal acidification with 0.1 N HCl significantly increased pyloric pressure. Atropine 15 mug per kg, subcutaneously prevented the rise of pyloric pressure in response to acid infusion into the duodenum.