Direct effects of 5-(3-tert.-butylamino-2-hydroxy) propoxy-3,4-dihydrocarbostyril hydrochloride (carteolol) on chronotropic and inotropic activity were investigated in seventeen isolated atrium preparations which were suspended in a bath and perfused with arterial blood from the heparinized donor dog. Carteolol has little adrenergic beta-receptor stimulating effect in doses which block prominent positive chronotropic and inotropic effects of norepinephrine. At relatively larger doses, however, carteolol produced a long-lasting positive chronotrophic and inotropic effect. At extremely larger doses, carteolol produced a negative inotropic effect with little or slight negative chronotropic effect. In propranolol-treated preparations, carteolol induced less positive effects. Propranolol-induced negative chronotropic and inotropic effects were significantly reduced by treatment with carteolol. From these results, carteolol has not only potent beta-adrenoceptor blocking property but also sympathomimetic activity, and different beta 1-type cardiac adrenergic receptors may probably be suggested.