Each subject will undergo a brief history and physical exam prior to entering the study. The subject will have height and weight measured. A questionnaire will be administered to the subject inquiring about gastrointestinal symptoms and medical history. This is to screen for a history of underlying pulmonary, cardiovascular, or gastrointestinal disease. Females will be studied only during the follicular phase (days 1-10) of the menstrual cycle, to minimize the effect of the menstrual cycle on motility (Wald, at al). In addition, females will be given pregnancy screening with a urine beta-human chorionic gonadotropin pregnancy test (Sure-Vue; Fisher Scientific, Inc) prior to the start of the study.