Inclusion Criteria (HIV-infected group):  HIV-positive Good medication adherence Negative urine toxicology for drugs of abuse (including cannabis) Good mental and physical health (other than HIV) as determined by history, psychiatric interview, collateral information, physical and laboratory examinations, ECG, and vital signs.  Inclusion Criteria (HIV-uninfected group):  HIV-negative Negative urine toxicology for drugs of abuse (including cannabis) Good mental and physical health as determined by history, psychiatric interview, collateral information, physical and laboratory examinations, ECG, and vital signs.  Exclusion Criteria (common to both groups):  Major medical conditions, e.g., myocardial infarction, hypertension, etc. Positive pregnancy test and lactation; Weight greater than 166 kg (365 lbs)