This study will include 224 subjects, who must be men and women between the ages of 13 and 45. Half the subjects will be healthy participants, and half will have had chronic abdominal pain of unknown origin for longer than 6 months. Female participants must take a urine pregnancy test before starting the study, and will not be allowed to participate if the test is positive. During the study, patients will visit the NIH Clinical Center on two occasions for testing.