A Phase 1, multicenter, pharmacokinetic/Pharmacodynamic and safety study in which infants will be randomized in an open-label fashion to receive 5 days of open-label treatment with lansoprazole pediatric suspension 1 mg/kg/day oral, or lansoprazole pediatric suspension 2 mg/kg/day oral. On Dosing Days 1 and 5, blood samples will be obtained for drug assay. All subjects will be evaluated for inclusion in the pH monitoring portion of the study and will undergo pH monitoring, provided it is clinically indicated, at the discretion of the investigator. Intragastric pH monitoring (up to 24 hours) will be performed at Baseline, on Dosing Day 1 (or Day 2) and on Dosing Day 5 (or Day 6). Intraesophageal pH may be done in addition to intragastric pH at the discretion of the investigator. Subjects will be evaluated for safety, including a follow-up visit on Post-Dosing Day 14.