Cohort 1: Women in active labor receive 3 doses of oral ZDV 3 hours apart followed by PK sampling. After PK sampling post 3rd oral dose, or at 8 cm dilation (whichever comes first), women receive IV ZDV until delivery. The cord blood will be analyzed for serum ZDV PK levels and the infant's infection status will be determined. If the dose of ZDV is adequate to achieve targeted maternal serum drug levels in Cohort 1 and if oral dosing is tolerated in this cohort, Cohort 2 will begin enrollment. [AS PER AMENDMENT 08/03/01: Cohort 1 has been completed and only Cohort 2 is open for enrollment.] Cohort 2: [AS PER AMENDMENT 08/03/01: Women in active labor receive initial loading dose of oral ZDV, then another dose of oral ZDV 3 hours later, both doses followed by PK sampling. After PK sampling post second oral dose, or at 8 cm dilation (whichever comes first), woman receives IV ZDV until delivery. The cord blood will not be analyzed for serum ZDV PK levels and the infant's infection status will be determined.]