We developed and validated a liquid chromatography-mass spectrometry (LC-MS/MS) method for detecting conivaptan, an arginine vasopressin receptor blocker, in human plasma to facilitate future pharmacokinetic studies. After acetonitrile protein precipitation, analytes were separated on a Grace Alltima HP C18 reverse-phase column (5 mu m, 2.1 x 50 mm), and gradient elution was performed using 0.1% formic acid and 5 mM ammonium formate in acetonitrile-water (1:9) and acetonitrile-water (9:1) over a 4-min run time. Positive-mode electrospray ionization and multiple reaction monitoring were applied. The assay was linear over the concentration range of 1-500 ng mL-1, with confirmed accuracy (intra-batch, 0.5-1.1%) and precision (intra-batch relative standard deviation [RSD], <= 5.7%; inter-batch RSD, <= 6.8%). Extraction recovery varied between 94.2-100.5%, and the matrix effect was 94.9-97.0%. To establish the stability, we examined plasma for 17 h at room temperature, 21 h at 2-8 degrees C, 57 days at -70 degrees C for three freeze-thaw cycles, and after processing for 52 h in the autosampler. We detected acceptable stability under all examined conditions. Hemolysis, hyperlipidemia, and dilution integrity satisfied the validation criteria. The method developed in this current study will help accurately quantify conivaptan in human plasma and define its clinical pharmacokinetics in the future.