Methadone maintenance programs are considered the standard of care for the pregnant opiate addict. However, data on changes in methadone pharmacokinetics (PK) during pregnancy are limited and do not include its disposition by the placenta due to obvious ethical and safety considerations. Accordingly, investigations in our laboratory are focusing on human placental disposition of opiates including methadone. Recently, we reported on methadone metabolism by placental aromatase and provide here data on its bidirectional transfer across the tissue utilizing the technique of dual perfusion of placental lobule. The concentrations of the opiate transfused into the term placental tissue were those reported for its in vivo levels in the maternal serum of women under treatment with the drug. Data obtained indicated that the opiate has no adverse effects on placental viability and functional parameters and that it is retained by the tissue. Also, methadone transfer and its clearance index in the fetal to maternal direction (0.97+/-0.05) was significantly higher (P<0.05) than in the maternal to fetal (0.83+/-0.09). The observed asymmetry in methadone transfer could be explained by the unidirectional activity of the efflux transporter P glycoprotein (P-gp) that is highly expressed in variable amounts in trophoblast tissue. Therefore, placental disposition of methadone might be an important contributor to the regulation of its concentration in the fetal circulation and consequently may affect the incidence and intensity of neonatal abstinence syndrome for women treated with the drug during pregnancy.