background: Methotrexate (MTX), an antifolate agent, is primarily eliminated by the kidney. Organic anion transporter 3 (OAT3) contributes to renal MTX clearance. Several studies have shown an association between co-administration of proton pump inhibitors (PPIs) and delayed elimination of MTX, but the findings are conflicting. In this study, we aimed to evaluate whether the differential inhibitory effects of PPIs on the OAT3-mediated transport of MTX are associated with the risks of delayed MTX elimination. methods: We investigated the effects of PPIs on rat (r) OAT3-mediated MTX uptake using HEK293T cells expressing rOAT3. To examine whether PPIs could affect the pharmacokinetics of MTX, changes in plasma concentration-time profiles were assessed when MTX (50 mg/kg, ip) and a range of PPIs (2 mg/kg, iv) were administered to rats. results: In vitro studies demonstrated that PPIs inhibited rOAT3-mediated uptake of MTX, with estimated IC values of 2.1-5.2 μM, and a rank order of esomeprazole ≈ lansoprazole ≈ omeprazole > rabeprazole. When MTX and esomeprazole were co-administered to rats, the plasma concentration of MTX 6 h after administration and the t were significantly higher than those in the vehicle group. The effect of lansoprazole was not significant, but showed a tendency to prolong plasma MTX levels. Famotidine, a histamine H-receptor antagonist, showed a weak inhibitory effect on rOAT3-mediated MTX uptake, although it did not affect plasma concentration-time profile of MTX in vivo. conclusions: Esomeprazole increases the t of MTX in rats, which may be partially attributed to the inhibition of rOAT3.