HIV infected patients, aged </= 35 years and aged >/= 55 years, who are regularly taking either efavirenz or lopinavir/ritonavir as part of their antiretroviral regimen are being asked to spend 12 hours at our clinic (the ACS clinic at Jacobi Medical Center, Bronx, NY) for an intensive pharmacokinetic study in which 5 cc of blood will be drawn 9 times over a 12 hour period. Patients taking EFV will return for one additional blood draw the following day.

Patients will be asked to come to the clinic at 7 am and bring their medication to the clinic. Blood will be drawn to obtain a trough level of either EFV or Lop/r after which the patient will be instructed to take their EFV or Lop/r. Then blood will be drawn 1 hour, 2 hours, 3 hours, 4 hours, 6 hours, 8 hours, 10 hours and 12 hours later. Patients on EFV will be asked to return the next morning for a 24 hour blood level.

The blood will be spun and the plasma frozen at - 70 C. The plasma samples, labelled with the patients' initials and the draw date, will be shipped to University of Alabama, Department of Pharmacology, where the drug levels will be measured.

Patients will be recruited from the ACS outpatient clinic. They will be identified by their providers.