SummaryAdministration of zidovudine (ZDV) to pregnant women with human immunodeficiency virus infection may be of benefit to both the mother and the unborn child. Before testing this hypothesis, however, it is necessary to determine the transplacental transfer, fetal toxicity, and fetal accumulation of ZDV (if any) in a representative animal model. Therefore, the transplacental transfer and the fetal accumulation of ZDV were determined at steady state in near-term pregnant macaques (Macaca nemestrina). ZDV was administered to five dams at a rate predicted to produce a steady-state plasma concentration of about 1 μg/ml. When steady state was predicted to have been achieved, a cesarean section was performed on each dam. At this time, blood samples from the dam (peripheral vein) and the fetus (umbilical vein) were obtained simultaneously. The plasma concentration of ZDV and its major metabolite, zidovudine glucuronide (ZDVG), were determined by a specific high-performance liquid chromatography (HPLC) method. The ratio of steady-state plasma concentration (Crss) of ZDV in the fetus (Cssf) to that in the dam (Cssd) (Crss=Cssf/Cssd) was found to be close to unity (0.826 ± 0.067). Similar results were obtained for the ratio of steady-state unbound ZDV plasma concentration (0.852 ± 0.083). We conclude that ZDV readily crosses the placenta, probably by passive diffusion, and that ZDV does not accumulate in the fetus when administered to near-term pregnant macaques.