Subjects may also be asked several days prior to the study to report to the Clinical Research Center for a "mock" study to see if they are comfortable with the MRI scan and/or to test their body's response to either protocol (as described above, but carried out at the Clinical Research Center). If subjects would like to try the MRI procedure in advance, this test will not exceed 2 hours. If we need to test the subject's response to the infusion, this "pre-test" will involve only drawing blood and administering glucose and will be performed as described for either protocol above, but will last not more than 4 hours.