Patients receive AZT by mouth on the first day. After taking the AZT, blood samples are taken from a catheter and several urine samples are collected over a 24-hour period. During this time, patients remain in the hospital for the 24 hours or may choose to go home 12 hours after taking the AZT dose and return for the last blood sample the next morning. Following study day 1, patients receive AZT every 4 hours, including in the middle of the night, and keep a diary of the times they take AZT, as well as of the use of other medications, tobacco, or alcohol. A return appointment is made for 8-15 days later. On that day, patients again receive AZT by mouth, and blood tests and urine samples are again taken. Patients who are receiving hemodialysis participate in 1 additional day of pharmacokinetic studies to be arranged during one hemodialysis session. Patients on Continuous Ambulatory Peritoneal Dialysis (CAPD) are studied separately and do not participate in the procedures for the other groups. AZT is given as a single oral dose at the beginning of the first morning exchange followed by a pharmacokinetic study. Chronic AZT dosing is initiated following the first exchange. After a minimum of 7 days of AZT therapy and a maximum of 14 days the last dose of AZT is administered and a repeat pharmacokinetic study is done. All patients are seen again 1-2 weeks after completing the last pharmacokinetic study.