P-glycoprotein is an ATP-dependent drug transport protein that is predominantly found in the apical membranes of various epithelial cell types in the body, including the blood luminar membrane of the brain capillary endothelial cells that make up the blood-brain barrier. Increased P-glycoprotein expression in the blood-brain barrier has been described in epileptogenic brain tissue of patients with pharmacoresistant epilepsy, suggesting that overexpression of P-glycoprotein may be involved in multidrug resistance of epilepsy. The mechanisms underlying the overexpression of P-glycoprotein in brain tissue of epileptic patients are not clear. Two antiepileptic drugs, phenobarbital and phenytoin, have been reported to up-regulate P-glycoprotein in cell cultures, so that chronic treatment with antiepileptic drugs may enhance P-glycoprotein expression in the blood-brain barrier. To directly address this possibility, we treated rats with phenobarbital or phenytoin over 11 days and subsequently determined expression of P-glycoprotein by immunohistochemistry in endothelium and parenchyma of several brain regions, including regions of the temporal lobe, which is often involved in pharmacoresistant types of epilepsy. Except for a moderate increase in the intensity of P-glycoprotein expression in the piriform/parietal cortex and cerebellum of phenobarbital-treated rats, no significant P-glycoprotein increases were seen after prolonged treatment with phenobarbital or phenytoin in any brain region examined. In view of recent findings that seizures lead to a transient induction of P-glycoprotein in the brain of rats, it seems reasonable to suggest that the overexpression of P-glycoprotein in brain regions of patients with intractable epilepsy is a consequence of uncontrolled seizures rather than of chronic treatment with antiepileptic drugs.