Effects of octopamine on sinus rate and atrial contractility were investigated using the isolated atrium preparation of the dog which perfused with heparinized arterial blood led from a support dog. When octopamine, dopamine or norepine-phrine was administered into the cannulated sinus node artery, positive chrono-tropic and inotropic responses were dose-relatedly induced from 0.1μg, 0.1μg or 0.01μg, respectively. The D. R. 50 values (dose ratio at 50% maximum response) of octopamine, dopamine and norepinephrine are roughly 30-100: 30: 1, respectively. The duration of action of octopamine was longest. Effects induced by octopamine were blocked by an adrenergic beta-blocking agent, alprenolol. Desmethyl-imipramine treatment significantly suppressed octopamine-induced effects but rather enhanced norepinephrine-induced ones. Octopamine-induced effects were not influenced by tetrodotoxin which blocked those induced by nicotine. From these results, it is concluded that positive chronotropic and inotropic responses to octopamine are mainly due to a tyramine-like action.