S-Tenatoprazole-Na (STU-Na), a new drug currently under clinical development, belongs to a class of drugs, called proton pump inhibitors (PPls). Some PPIs are already commercially available. STU-Na will be used for treatment of acid related diseases (gastroduodenal ulcers, erosive or ulcerative esophagitis due to gastroesophageal reflux disease). This study evaluates the degree of acid suppression by different doses of STU-Na. The degree of acid suppression is considered to be correlated with clinical efficacy.