In the present investigation, we analyzed the cytotoxicity of extracts of these plants towards sensitive and multidrug-resistant, P-glycoprotein-expressing tumor cells. We further examined the inhibition of P-glycoprotein in porcine brain capillary endothelial cells. P-glycoprotein represents a major key player for the failure of chemotherapy of tumors and drug transport throughout the body. The potential of Thai medicinal plants to treat multidrug-resistant tumors and to inhibit P-glycoprotein is largely unknown.