To 60 ml of anhydrous dimethylformamide solution containing 6.7 g of 6-(α-chloroacetyl)-3,4-dihydrocarbostyril was added 11.5 g of 4-(3-chlorobenzoyl)piperazine and 5 ml of triethylamine and the whole mixture was stirred at 50 to 60° C. for 1 hour. The reaction mixture was poured into a large amount of water, and the organic layer was extracted with chloroform. The chloroform layer was washed with water, dried, then chloroform was removed by evaporation. The residue thus obtained was recrystallized from methanol-chloroform to yield 5.5 g of 6-[-4-(3-chlorobenzoyl)-1-piperazinylacetyl]-3,4-dihydrocarbostyril 1/2-hydrate in the form of colorless powdery substance. Melting point: 231°-234° C.
