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LJ-2698, a highly potent human A 3 adenosine receptor antagonist with nucleoside structure, was designed to have a minimal species dependence. For further pre-clinical studies, analytical method for the detection of LJ-2698 in rat plasma was developed by liquid chromatography-tandem mass. Plasma samples were processed by protein precipitation method with acetonitrile, using losartan as the internal standard (IS). Chromatographic separation was carried out using a Kinetex C18 column (100 × 4.6 mm; 100 Å; 2.6 μ) with acetonitrile/water with 0.2% (v/v) formic acid (65:35, v/v) in the isocratic mode at a flow rate of 0.4 mL/min. Mass spectrometric detection in multiple reaction monitoring mode was performed with positive electrospray ionization. The mass transitions of LJ-2698 and IS were m/z 412.3 → 294.1 and m/z 423.1 → 207.2, respectively. The calibration curves were linear in the range 5.00-5000 ng/mL (r 2  ≥ 0.998). The lower limit of quantification was established as 5.00 ng/mL. Within- and between-run precisions were <7.01%, as relative standard deviation; and accuracies were in the range 3.37-3.64%, as relative error. The validated method was successfully applied to its pharmacokinetic evaluation after intravenous and oral administration in rats, and the dose-dependent pharmacokinetic behavior of LJ-2698 was elucidated for the first time.