Patients are hospitalized overnight on three separate occasions. On the first admission, AZT is administered every 4 hours. On the second day, 15 blood samples are taken to determine how fast the AZT enters and is removed from the bloodstream (pharmacokinetics). On the second day, after all the blood specimens have been collected, patient starts taking probenecid by mouth every 8 hours, and is discharged from the research unit. The AZT dose is then taken every 8 hours. One week later and again 3 weeks after that, patient is readmitted overnight and the blood sampling to measure AZT levels is repeated. AMENDED: 8 additional patients will be enrolled using the same doses of AZT but a lower dose of probenecid.