Inclusion Criteria

Participants in Groups 1 and 2 may be eligible for this study if they:

• Are at least 18 years old.

Participants in Group 1 (HIV-uninfected) may be eligible for this study if they:

• Are HIV-uninfected within 30 days prior to study entry.

Participants in Group 2 (HIV-infected) may be eligible for this study if they:

• Are HIV infected.
• Have been taking an NRTI-containing anti-HIV drug regimen for 8 weeks or more prior to study entry.
• Have a nonexercise venous lactate level greater than 2 times the upper limit of normal (ULN) on 2 repeated measurements.

Exclusion Criteria

Participants in Groups 1 and 2 may not be eligible for this study if they:

• Have severe claustrophobia.
• Have severe symptoms that, in the opinion of the investigator, would interfere with the ability of participants to perform the exercise required for spectroscopy testing.
• Have hepatitis C or B, within 90 days prior to study entry.
• Have taken certain drugs within 30 days prior to study entry.
• Have a medical condition associated with chronic liver disease other than hepatitis C and B.
• Have consumed excessive amounts of alcohol in the past 12 months.
• Are pregnant or breast-feeding.
• Have a foreign object or metal in their body that would impair MRS testing.
• Have or have had peripheral vascular disease.
• Weigh 250 lbs or more.
• Have a family history of mitochondrial disease or have skeletal muscle disease, heart muscle disease, or nervous system conditions without a clearly defined cause that is not related to mitochondria.

Participants in Group 1 (HIV-uninfected) may not be eligible for this study if they:

• Have any active medical condition.
• Use any prescription drugs, except for vitamins or oral contraceptives.