Sumatriptan (Imitrex), a selective 5-hydroxytryptamine receptor agonist, has been found to be of therapeutic benefit in the acute management of migraine. There is no information on the transfer of this agent across the human placenta. Accordingly, the current study assessed the transport of this drug across the normal term human placenta, using the isolated perfused single cotyledon technique. We found that only about 15% of a single dose of the agent placed in the maternal reservoir crossed into the fetal compartment over 4 hr. Given the average elimination half-life of 2 hr for sumatriptan, it is evident that only very small amounts of the agent will cross from mother to fetus after single doses of Imitrex. Only the parent drug entered the fetal compartment. Metabolites were not detected in the perfusates, but there was evidence of some metabolism of sumatriptan in the placenta. The nature of the metabolites has not been determined. The mechanism of transfer of the drug across the placenta is passive (i.e., the clearance is similar to L-glucose which is passively transported), the rate of transfer is equal in both directions (maternal to fetal and in the reverse), and the drug does not cross into the fetus against a concentration gradient. This passive transport of sumatriptan across the placenta is consistent with its molecular weight, its water solubility, and its slow penetration across the blood-brain barrier in experimental animals.