Ten patients with Zollinger-Ellison syndrome (4 with intact stomach and 6 with previous partial gastrectomy) received ranitidine (600-1200 mg daily) for 2-24 months (mean 14 months). The inhibitory effect of ranitidine on acid secretion was evaluated by measuring acid output during the third hour following a single drug administration in all cases, and by the 24-hour intragastric pH profile in 5 patients. Therapeutic efficacy was assessed by clinical and endoscopic criteria. Ranitidine at different dosages, related to the rate of acid secretion, was effective in controlling peptic disease and diarrhoea in the majority of patients. The secretory parameters used in this study allowed evaluation of the inhibitory effect of ranitidine on acid secretion, in order to select the best therapeutic dose and to decide on surgical cure in the patients non-responsive even to the highest dosage.