After 6 days of inpatient detoxification with clonidine, patients addicted to opiates are randomized to receive either oral or intravenous AZT for the first dose, followed by determination of plasma and urine pharmacokinetics. On the second day of AZT dosing, the alternate form of administration will be used for the first dose. On both days, all other doses are given orally. Patients then begin methadone maintenance in combination with AZT for 7 days of inpatient treatment, with further pharmacokinetic sampling. After hospitalization for 16 days total, patients continue AZT/methadone treatment on an outpatient basis, and then 2 months later are readmitted as inpatients for 5 days for further pharmacokinetic sampling. Control patients who are not addicted to opiates are hospitalized for 3 days at study entry and are randomized for AZT treatment and pharmacokinetic sampling in the same manner as the first group, although they will not receive methadone treatment. Control patients are readmitted for 2 days after 1 week of AZT treatment and then again after 59 days of AZT treatment.