Since zidovudine and ddI may be used in combination in the future to treat pregnant women who are human immunodeficiency virus positive, we conducted a study to determine whether zidovudine affects the transfer of ddI across the placenta. Zidovudine and ddI were infused simultaneously to three near-term pregnant macaques (Macaca nemestrina) at 156 +/- 1.5 days of gestation. Samples of maternal and fetal blood and amniotic fluid were drawn at intervals for 30 h. The steady-state dideoxyinosine concentrations in the plasma of the dam (Cssd), the fetus (Cssf), and the amniotic fluid (Cssa) and the ratios Cssf/Cssd and Cssa/Cssf were found to be not significantly different from the values previously determined after the administration of ddI alone during the same pregnancy. We conclude that concurrent zidovudine administration does not affect the transfer of ddI across the placenta in near-term Macaca nemestrina.