Patients receive a single IV dose of ddI on day 1 and pharmacokinetics values are determined. At least 48 hours but no more than 1 week after the IV dose, patients receive an oral dose, and oral pharmacokinetics are obtained for 8 hours. Oral ddI is then administered every 12 hours until labor commences and then after delivery, every 12 hours until 6 weeks postpartum. During labor and delivery, patients receive a loading dose of ddI followed by continuous infusion. Pharmacokinetics are obtained during infusion and also at 6 weeks postpartum. AS PER AMENDMENT 11/24/97: Maternal IV pharmacokinetic studies will not be performed after ddI IV formulation has expired (11/30/97). Maternal and cord samples will be required regardless of whether mother has received continuous IV ddI during labor.