For the purpose of investigating drug interactions, a new selective method for determination of 5-dimethylamino-9-methyl-2-propyl-1H-pyrazolo[1,2-a] [1,2,4]benzotriazine-1,3(2H)-dione-dihydrate (azapropazone, Prolixan 300) was developed. The method is based upon the direct quantitation of the drug by thin-layer chromatography using remission measurement. The method is well suited for routine analysis of numerous samples, because of its simplicity and rapidity. The standar deviation of the method is about +/-4% at therapeutic plasma concentrations.