During labor, patients continue to receive AZT intravenously every 4 hours until they deliver their babies. Blood and urine tests are made to measure the amounts of AZT in the mother's and, after delivery, the baby's body fluids. AZT is stopped after the baby has been delivered. The initial enrollment is six women with no history of intravenous drug abuse plus another four women with a history of intravenous drug use, who have been receiving methadone maintenance therapy. These women are not required to have a negative urine toxicology screen for illicit drugs to enter the study.