The purpose of this study was to investigate the modification of expression and functionality of the drug transporter P-glycoprotein (P-gp) by tumor necrosis factor-alpha (TNF-${alpha}$) and interferon-gamma (IFN-${gamma}$) at the blood-brain barrier (BBB). We used immortalized human brain microvessel endothelial cells (iHBMEC) and primary human brain microvessel endothelial cells (pHBMEC) as in vitro BBB model. To investigate the change of p-gp expression, we carried out real time PCR analysis and Western blotting. To test the change of p-gp activity, we performed rhodamin123 (Rh123) accumulation study in the cells. In results of real time PCR analysis, the P-gp mRNA expression was increased by TNF-${alpha}$ or IFN-${gamma}$ treatment for 24 hr in both cell types. However, 48 hr treatment of TNF-${alpha}$ or IFN-${gamma}$ did not affect P-gp mRNA expression. In addition, co-treatment of TNF-${alpha}$ and IFN-${gamma}$ markedly increased the P-gp mRNA expression in both cells. TNF-${alpha}$ or IFN-${gamma}$ did not influence P-gp protein expression whatever the concentration of cytokines or duration of treatment in both cells. However, P-gp expression was increased after treatments of both cytokines together in iHBMEC cells only compared with untreated control. Furthermore, in both cell lines, TNF-${alpha}$ or IFN-${gamma}$ induced significant decrease of P-gp activity for 24 hr treatment. And, both cytokines combination treatment also decreased significantly P-gp activity. These results suggest that P-gp expression and function at the BBB is modulated by TNF-${alpha}$ or/and IFN-${gamma}$. Therefore, the distribution of P-gp depending drugs in the central nervous system can be modulated by neurological inflammatory diseases.