Healthy normal volunteers between 18 and 40 years of age may be eligible for this study. Candidates must be HIV-negative and have no history of heart disease, hypertension, or diabetes mellitus. They must not have smoked for at least 6 weeks before entering the study. Volunteers will be screened with a medical history, physical examination, blood tests (including a pregnancy test for women of childbearing potential), and electrocardiogram. In addition, candidates will have an oral glucose tolerance test (see description below).