3-[2-(4-PHENYL-1-PIPERAZINYL)ETHYL]-4H-pyrido[1,2-a]pyrimidin-4-one derivatives have central nervous depressant and hypotensive activities and are useful as major tranquillizers. The compounds can be prepared by (a) reacting 2-methyl-3-halogeno (or arylsulfonyloxy or alkylsulfonyloxy)ethyl-4H-pyrido[1,2-a]pyrimidin-4-one derivative with 1-phenylpiperazine derivative or (b) reacting an aminopyridine with 2-[2-(4-phenyl-1-piperazinyl)ethyl]-acetoacetic acid ester.