Patients with bleeding ulcers identified by endoscopy will be randomly assigned to receive an acid-blocking drug (called a proton pump inhibitor [PPI]) either by mouth every 3 hours for 24 hours or intravenously (IV) by constant infusion for 24 hours. A pH probe in the stomach will be used to determine intragastric pH (a measure of the acid production in the stomach) at baseline and during the 24 hours of therapy. The purpose of the study is to determine if the continuous intravenous administration of the drug provides better reduction of acid in the stomach than the oral administration.