Concurrent treatment with methotrexate (MTX) and antiepileptic drugs, such as phenobarbital (PB), reduces the efficacy of MTX chemotherapy in childhood acute lymphoblastic leukemia (ALL). This can result from defective Reduced folate carrier (Rfc1)-dependent cellular uptake of MTX. Indeed, we have shown that functional Rfc1 activity is significantly reduced by clinically relevant concentrations of the anticonvulsant drugs PB or carbamazepine in an adequatein vitromodel. As PB is known to regulate carrier-associated transport by the nuclear receptor constitutive androstane receptor (CAR), we investigated the involvement of the CAR signaling cascade and the mode of PB-induced downregulation of Rfc1 activity. CAR activation by PB or the CAR agonist 1,4-bis[2-(3,5-dichloropyridyloxy)]-benzene resulted in translocation of Ca2+-dependent protein kinase Cα (cPKCα) to the plasma membrane related to significantly elevated PKC activities. In contrast, subcellular localization of Ca2+-independent PKCδ was only marginally altered. Studies on intracellular distribution of the Rfc1 protein indicated that PB-induced activation of cPKCα was associated with carrier internalization from the plasma membrane into the cytosol independent of the Rfc1 phosphorylation status. In conclusion, we identified for the first time the molecular mechanism of this clinically relevant drug resistance in patients with ALL concurrently receiving MTX chemotherapy and antiepileptic drugs.