Group 1 will receive capravirine alone for 7 days and capravirine plus efavirenz (another NNRTI) for an additional 10 days. On day 1, participants will have a physical examination, urine test and laboratory blood tests. Blood samples will be collected just before the first dose of capravirine is given and again at 0.5, 1, 2, 4, 6, 8 and 12 hours after the dose. A small plastic tube will be placed in a vein to avoid multiple needle sticks. On day 8, participants will have another physical examination and laboratory blood tests. Blood samples will be drawn again as described above. In the evening of day 8, efavirenz will be added to the regimen. On day 18 (the last day of the study), participants will have another physical examination and blood tests. Blood will again be collected as described above.