On day 21, all patients will report to the GI procedure room three hours prior to their endoscopy. An intravenous catheter will be placed for the procedure. At 2.5 hours prior to the procedure, patients will receive two, 5 mg montelukast tablets with 2 oz of water and the time will be recorded. Blood samples will be obtained at 1, 2.5, and 6 hours post-dosing. Plasma will be moved following centrifugation and stored at -70ºC. At the time of endoscopy (approximately 2.5 hours post-dosing), multiple duodenal mucosal biopsies (appx. aggregate tissue yield = 0.5 gm) will be obtained for determination of tissue drug concentration. All serum and tissue drug determinations will be determined by the manufacturer (Merck) using a validated HPLC method that has been used to support two previous pharmacokinetic studies of montelukast (9,10) conducted in pediatric patients, both of which have successfully used the aforementioned approach for pharmacokinetic data analysis.