A procedure for the quantitative determination of adinazolam in plasma was developed. The drug, an N-demethylated metabolite, and an internal standard were extracted from basified plasma into ethyl acetate. After evaporation, the residue was dissolved in toluene which was washed with sodium hydroxide. The toluene was evaporated and the residue was dissolved in a mixture of acetonitrile, methanol, and water for chromatography. The concentrations of the drug and the metabolite were determined using reverse-phase liquid chromatography with UV detection at 254 nm. The assay methodology showed good peak height ratio-concentration linearity, precision, and accuracy and has been used to analyze plasma samples collected from human subjects after oral administration of adinazolam mesylate in compressed tablets. The low plasma background interferences allowed the quantitative determination of concentrations as low as approximately 5 ng/mL.