It is well-known that flavonoids including rutin exert a wide range of pharmacological effects. This study was carried out for further characterization of the pharmacological effect of rutin. A significant rise in the serum 5-HT levels was found 2 h after the administration of rutin (25 mg kg-1 i.p.). The same dose of rutin caused no significant change in the serum 5-hydroxytryptomine levels after 8 and 72 h when compared to controls. In the in vivo experiments, rutin (25 mg kg-1 i.p.) was ineffective on the blood pressure and respiratory tract of the guinea-pigs in spite of its smooth muscle relaxant effects in the in vitro preparations. These findings strongly suggest a physiological antagonism in the effects of rutin in intact animals.