• Pharmacokinetic parameters [ Time Frame: At Days 1, 5, and 10 ] [ Designated as safety issue: No ]
  
• Pharmacodynamic parameters [ Time Frame: At Days 1, 5, and 10 ] [ Designated as safety issue: No ]
  

• Number of Patients with Adverse Events as a Measure of Safety [ Time Frame: Approximately 9 weeks ] [ Designated as safety issue: Yes ]
  

This is an open-label (both physician and patient know the name of the study medication), multicenter, Phase I study, consisting of 2 parts. The first part of the study will be nonrandomized (study drug is intentionally assigned), and all patients will receive the same dose. In the second part of the study, patients will be randomized (study medication assigned by chance) into 2 dose groups. The purpose of the study is to evaluate the pharmacokinetics (PK), pharmacodynamics (intraesophageal/intragastric pH, clinical global impression, formulation palatability and Gastroesophageal Reflux Disease (GERD) daily symptom diary) and safety of rabeprazole after single and multiple daily administration at 2 dose levels in children between the ages of 1 and 11 months, inclusive (up to 11 months 29 days), with GERD. As this study is an exploratory assessment of the PK, pharmacodynamics and safety of rabeprazole in children, no formal hypothesis testing is applied. Safety and tolerability, including monitoring of adverse events, clinical laboratory results, physical examination, vital signs and electrocardiogram measurements (the measuring of the electrical currents in the heart), will be evaluated throughout the study. Patients will receive rabeprazole sodium as single daily oral doses for 5 successive days (Option 1), or up to 14 successive days (10 days minimum) (Option 2) as a bead formulation. In Part 1, patients will receive single and multiple daily (every 24 hours) doses of 0.5 mg/kg, using increments of 1 mg dose. Safety and PK data from Part 1 of the study will determine the 2 dosages to be studied in Part 2 of the study. In Part 2, patients may receive rabeprazole sodium as single daily oral doses for up to a maximum of 28 consecutive days as a bead formulation. Patients will be randomized into either the 5 mg dose or 10 mg dose group.