Parenteral control of gastric acid hypersecretion in patients with Zollinger-Ellison syndrome is increasingly required; however, existing methods of determining the required dose are cumbersome and not applicable in all centers. A previous study suggested that the required parenteral dose of histamine H₂-receptor antagonists correlated with the previous oral dose. In the present study, in 31 patients with Zollinger-Ellison syndrome we evaluated the hypothesis that an effective parenteral histamine H₂-receptor antagonist dose could be predicted from the previous oral dose. Twenty-three patients were taking oral ranitidine (mean 1.3 g/day), six patients famotidine (152 mg/day), and two patients cimetidine (1.8 g/day). Each patient was treated with a continuous intravenous infusion of the equivalent dose of ranitidine (mean dose 1 mg/kg/hr with 35% requiring 0.5 mg/kg/hr, 49% 1 mg/kg/hr, 3% 1.5 mg/kg/hr, 10% 2 mg/kg/hr, and 3% 2.5 mg/kg/hr. This dose of ranitidine acutely controlled acid secretion (<10 meq/hr)="" in="" all="" patients.="" to="" evaluate="" long-term="" efficacy="" and="" safety,="" 20="" patients="" were="" maintained="" on="" this="" dose="" through="" the="" peri-and="" postoperative="" periods.="" mean="" duration="" was="" 7.1="" days="" with="" 25%="" treated="" 3–5="" days,="" 40%="" 6–8="" days,="" 30%="" 8–10="" days,="" and="" 5%="">10 days. The predicted dose continued to control acid secretion in 95% of patients with one patient requiring one dose adjustment. No biochemical, clinical, or hematological toxicity was seen, although ranitidine was stopped in one patient because of skin rash. These results demonstrate that the parenteral dose of ranitidine required to control acid secretion in patients with Zollinger-Ellison syndrome can be predicted from the oral dose. This predicted dose will be acutely effective in all patients in reducing acid secretion to <10 meq/hr,="" the="" established="" level="" of="" control,="" will="" remain="" effective="" in="" 95%="" of="" patients="" for="" up="" to="" 11="" days,="" and="" is="" safe.="" by="" utilizing="" the="" oral="" dose="" to="" predict="" the="" intravenous="" dose,="" repeated="" dose="" titrations="" will="" be="" avoided="" and="" thus="" this="" method="" should="" be="" generally="" useful="" in="" all="">