Oleoylethanolamide is an endogenous molecule with neuroprotective effects. It has been reported that exogenous oleoylethanolamide can be administered therapeutically, but the confounding presence of the endogenous molecule has led to conflicting reports regarding the mechanisms of the effects and highlights a need for an adequate methodology to differentiate them. We have developed a liquid chromatography-tandem mass spectrometry method to study oleoylethanolamide in rat plasma and brain using a 13 C-labeled isotope, 13 C-oleoylethanolamide. 13 C-oleoylethanolamide was extracted using a liquid-liquid extraction employing acetonitrile and tert-butyl methyl ether (1:4).Analysis was performed using a gradient with a total run time of 12 min. 13 C-oleoylethanolamide, d 4 -oleoylethanolamide (internal standard), and 12 Coleoylethanolamide (endogenous background) eluted simultaneously at 1.64 min. The method was validated for specificity, sensitivity, accuracy, and precision and found to be capable of quantification within acceptable limits of ±15% over the calibration range of 0.39-25 ng/mL for the plasma and 1.17-75 ng/g for the brain. It was then applied to quantify 13 C-oleoylethanolamide over 90 min after intravenous administration of a solution (1 mg/kg) in rats. Results suggest that 13 C-oleoylethanolamide does not reach therapeutic concentrations in the brain, despite a relatively prolonged plasma circulation, suggesting that rapid degradation in the brain remains an obstacle to its clinical application to neurological disease.