Upon study entry, volunteers receive APV plus EFV for 2 weeks. [AS PER AMENDMENT 12/4/00: Volunteers receive a single dose of APV alone on Day 0, EFV alone on Days 1 to 10, and APV combined with EFV on Days 11 to 13.] After 2 weeks [AS PER AMENDMENT 12/4/00: After completion of the second PK visit], volunteers are randomized to 1 of 5 treatment arms to add a second PI to the APV/EFV drug combination for 2 more weeks [AS PER AMENDMENT 12/4/00: for at least 1 week]. The treatment arms are as follows: