OBJECTIVE To evaluate the effectiveness of Ranitidine i.v. versus Omeprazole i.v. for gastrointestinal bleeding of peptic origin. METHODS Prospective, comparative, randomized and open study of 81 in-patients, hospitalized consecutively during 13 months, with gastrointestinal bleeding of peptic origin and signs of recent hemostasis (S.R.H.), belonging to the groups Forrest Ib and II. Treatment groups: group A: Ranitidine: initial injection of 50 mg. i.v., followed by 100 mg/6 hours i.v., during the first 72 hours and afterwards by 150 mg/12 hours, orally; group B: Omeprazole: initial injection of 80 mg. i.v., followed by 40 mg/8 hours i.v. during the first 72 hours and afterwards by 20 mg/24 hours, orally. Evaluation criteria: persistent haematemesis and melena; need for transfusions: treatment failure average period of hospitalization and disappearance of S.R.H. after 72 hours. RESULTS 43 patients received Ranitidine and 38 Omeprazole. Both groups were homogeneous in regard to variables ar hospitalization. No significant differences were found between these two groups: persistent melena (26% group A vs. 8% group B); patients who needed transfusion (39% vs. 31%); treatment failure (19% vs. 5%); average period of hospitalization; and disappearance of S.R.H. (81% vs. 95%). None of the patients died. CONCLUSIONS The effectiveness of Ranitidine i.v. and Omeprazole i.v., in the dosage used and in a selected group of patients with gastrointestinal bleeding, is similar. However, there is a trend to register less treatment failures and a higher percentage of S.R.H. disappearance with the patients treated with Omeprazole. Further studies with more patients are necessary to confirm this tendency.