1.8 grams of 1-methyl-5-hydroxy-3,4-dihydrocarbostyril and 1 g of NaH (50% in oil) are mixed with 30 ml of dimethylformamide. Then 2.6 g of 1-chloro-2-hydroxy-3-(4-phenylpiperazinyl)propane are added thereto at room temperature and stirred at 70° to 80° C. for 3 hours. The reaction mixture is poured into water and the organic layer is extracted with chloroform. After removing chloroform by distillation, the thus obtained residue is dissolved in acetone and the pH of the solution is adjusted by adding an acetone solution of oxalic acid. The crystals precipitated are colleced by filtration and recrystallization from methanol-acetone to obtain 2.8 g of 1-methyl-5-[2-hydroxy-3-(4-phenylpiperazinyl)-propoxy]-3,4-dihydrocarbostyril oxalate in the form of colorless crystals with a melting point of 220°-221° C. (decomp.).
