After more than 40 years of use, Praziquantel (PZQ) still remains the drug of choice for the treatment of intestinal and urogenital schistosomiasis. Its anti-parasitic activity resides primarily in the (R)-enantiomer. Hitherto neither the molecular target nor the pharmacokinetic-pharmacodynamic relationship have been fully elucidated. Here we investigated the efficacy and pharmacokinetics of PZQ in the Schistosoma mansoni mouse model to determine the key factors that drive its efficacy. Dose-response studies with racemic PZQ with or without addition of an irreversible pan-cytochrome P450 (CYP) inhibitor, 1-aminobenzotriazole (ABT), were performed. In addition, efficacy of PZQ in the presence of the CYP inducer, dexamethasone (DEX), was determined. Plasma samples were obtained by tail vein bleeding at 4 time points. The (R)-PZQ levels were determined using a LC-MS/MS method. Non-compartmental pharmacokinetic analysis was performed using PKsolver. In addition, experiments using an enhanced in vitro assay were conducted. We found that the use of ABT increased (R)-PZQ plasma exposures in the systemic circulation by ~10 to 20 fold but the latter were not predictive of efficacy. The use of DEX decreased plasma exposures of (R)-PZQ in the systemic circulation by ~10 fold without reducing efficacy. We extrapolated the (R)-PZQ concentrations in mouse portal vein / mesenteric veins from the systemic exposures and found that a free exposure of (R)-PZQ of ~ 20 μM*h in the portal vein was needed to obtain a worm burden reduction >60%. It is suggested that the high (R)-PZQ concentrations available before the hepatic first pass metabolism drive the efficacy against S. mansoni adult worms residing in the mesenteric veins. It is then possible that the current dosing regimen of 40 mg/kg in preventive chemotherapy programs may provide suboptimal concentrations in low-weight patients such as children, due to smaller total amounts of drug administered, and may consequently result in lower cure rates., Author summary After more than 40 years of use, Praziquantel (PZQ) still remains the drug of choice for the treatment of intestinal and urogenital schistosomiasis. However the relationship between pharmacokinetics and efficacy is not elucidated. For adults infected with intestinal schistosomiasis, a single oral dose of PZQ at 40 mg/kg is generally recommended by the World Health Organisation for preventive chemotherapy programs. By reducing hepatic metabolism with a pan-CYP inhibitor, we studied the systemic plasma exposure relationship versus activity in the Schistosoma mansoni mouse model. For S. mansoni infections where the worms are located in the mesenteric veins, we found that systemic plasma exposure was not predictive of efficacy, whereas estimated portal vein concentration of PZQ acts as a good predictor for the efficacy. This hypothesis was confirmed by pre-treatment of mice with a CYP inducer which led to a ~10 fold decrease in systemic plasma exposure while maintaining the efficacy of PZQ. This implies that the critical parameters are the rate and amount of PZQ absorbed, in order to achieve the required high local concentrations in the mesenteric veins. Therefore in pediatric patients the current dosing regimen of 40 mg/kg may provide suboptimal concentrations and consequently result in lower cure rates in S. mansoni infections.