The 18 patients are patients with AIDS or ARC. Nine are control patients who are not receiving methadone and are not IV drug abusers. Nine are former IV drug abusers who have been maintained on methadone for at least 6 months and who have been receiving a constant daily dose of methadone for at least 1 month. Plasma and urine levels of methadone are determined over a 4 hour period. The dose of AZT: For the pharmacokinetic study of AZT, plasma and urine samples are taken after oral dose of AZT and an intravenous dose of AZT in the control and methadone groups.