A test meal was given to 9 male controls and 9 male patients with duodenal ulcer. The serum gastrin concentrations were determined by radioimmunoassay, and gastric secretion of acid was determined before and after stimulation with 6 mug pentagastrin per kg body weight given subcutaneously. The pattern of gastrin response was found to be biphasic, with an early maximum of the first peak in the patients versus a monophasic response with a maximum in the second hour in the controls. Fasting serum gastrin levels were positively correlated to the basal acid outputs (BAO) in the patients but not in the controls. The mean quotient BAO/Fasting serum gastrin concentration was highest in the patient group, but the quotient of "maximal" acid output (MAO)-BAO/"integrated gastrin response" was equal in the two groups.