Patients receive their usual daily dose of methadone followed with ritonavir and saquinavir, respectively, twice a day. Patients are evaluated on Day 4 for safety and tolerance, and their ritonavir dose is increased. On Day 8 patients are evaluated for a steady-state level of methadone. After 2 weeks of the protease inhibitor therapy, they return for methadone pharmacokinetic sampling at Day 15 over 24 hours. Both protease inhibitors and methadone are administered on Day 15 on an inpatient basis. On Day 30, patients are assessed for safety and tolerance.