Although isoflurane is frequently used during hypothermic cardiopulmonary bypass (CPB), its pharmacokinetics in these circumstances have not been described for systems employing membrane oxygenators. The current study details isoflurane uptake and elimination in an ex vivo model of hypothermic CPB, which uses a Scimed membrane oxygenator. Isoflurane uptake and elimination by blood are markedly delayed by the membrane oxygenator in this system. This is because the oxygenator membrane absorbs large amounts of anesthetic; the Scimed oxygenator membrane has a capacity for anesthetic equivalent to approximately 17 L of blood. As isoflurane absorption by the oxygenator may also delay anesthetic uptake and elimination in patients during CPB, further studies in human subjects are needed to define the clinical significance of anesthetic absorption by the membrane oxygenator.