Vertical transmission of the human immunodeficiency virus 1 (HIV-1) is reduced from approximately 25% to approximately 7% as a result of 3'-azido-3'-deoxythymidine (AZT) therapy given during pregnancy; however, the consequences of transplacental AZT exposure to the fetus remain unknown. To address the extent and kinetics of AZT transfer across the human placenta, perfusion studies have been performed with fresh uninfected human placentas perfused with 0.5, 1. 0 and 5.0 mg AZT/ml for 2 h using a dual recirculating single cotyledon perfusion apparatus [T.I. Ala-Kokko, P. Pienimaki, R. Herva, A.I. Hollmen, O. Pelkonen, K. Vähäkangas, Transfer of lidocaine and bupivacaine across the isolated perfused human placenta, Pharmacol. Toxicol. 77 (1995) 142-148]. For two placentas, samples of perfusion effluent were taken every 15 min from the maternal and fetal sides of the apparatus and AZT levels were determined by AZT radioimmunoassay (RIA). At the end of the perfusion, AZT-DNA incorporation into placental DNA was determined by AZT-RIA. The concentration of AZT in the fetal perfusate increased with time, along with a concomitant slow decrease in the concentration of AZT in the maternal perfusates. For three different placentas, at 2 h after the start of perfusion, AZT-DNA incorporation values (molecules of AZT/10(6) nucleotides) were 11.8 for the 0.5 mg AZT/ml perfusate, 13.7 for the 1.0 mg AZT/ml perfusion, and 42.0 for the 5 mg AZT/ml perfusion. An additional placenta perfused with 1 mg AZT/ml did not have detectable values of AZT incorporated into DNA (data not shown). The data show that AZT crosses the human placenta and becomes rapidly incorporated into DNA of placental tissue in a dose-dependent fashion, suggesting that even short exposures to this drug might induce fetal genotoxicity and might also inhibit maternal-fetal viral transmission.