Pentamidine is used to treat Pneumocystis carinii pneumonia. The incidence of this infection in pregnancy has paralleled the increasing incidence of acquired immunodeficiency syndrome in pregnancy. Using the in vitro bidirectionally perfused human placenta, we studied the transfer of pentamidine across the placenta. Pentamidine was added to the maternal circulation at therapeutic concentrations (2 wg/ml). No transfer of pentamidine was detectable with a newly devised high-performance liquid chromatography method sensitive to 0.05 wg/ml of pentamidine. Increasing the pentamidine concentration tenfold produced a low level of transfer to the fetal circuit. Fetal concentrations were far below maternal perfusate concentrations. Placental tissue levels were higher than media levels. These data are suggestive of minimal drug transfer to the fetus and significant concentration of the drug in placental tissue. (Am J Obstet Gynecol 1989;160:759-61.)