Multidrug resistance (MDR) is a major contributory factor in the failure of chemotherapy. Concrete interpretation of P-glycoprotein (P-gp) substrate specificity, whether a substance is a substrate or an inhibitor, represents an important feature of a compound's pharmaceutical profiling in drug design and development. In this work, the P-gp substrate specificity of Maldi 531.2[M+H](+), a phenol ester from Aglaia loheri Blanco leaves was investigated. This study focuses on the effect of Maldi 531.2[M+H](+) on P-gp ATPase activity, which was examined by measuring the amount of inorganic phosphates (Pi) released as a result of ATP hydrolysis. To test the effects of Maldi 531.2[M+H](+) on MDR activity, an attempt to combine Maldi 531.2[M+H](+) with a potent P-gp substrate such as verapamil was performed. As a result of this combination treatment, two distinct patterns of interaction with P-gp activity were determined by a calcein-acetoxymethyl ester (AM) assay. Depending on the concentratgion, both stimulation and inhibition of MDR activity were observed at certain drug concentrations suggesting biphasic reactions, which can be understood as cooperative stimulation and competitive inhibition, respectively. Verapamil is a strong substrate to P-gp. Substrate specificity of Maldi 531.2[M+H](+) may be less than the substrate specificity of verapamil, but it acts additively together with low concentrations of verapamil in stimulating ATPase activity. On the one hand, verapamil and Maldi 531.2[M+H](+) exerted cooperative stimulation on P-gp. On the other hand, Maldi 531.2[M+H](+) acts as competitive inhibitor at higher concentrations.