In the isolated, blood-perfused canine atrium preparations, ethanol injected into the cannulated sinus node artery consistently produced dose-dependently negative inotropic effects at a dose range of 0.03 to 1 mmol and negative chronotropic effects starting from 0.1 mmol. Ethanol-induced negative inotropic and chronotropic responses were not blocked by an adequate dose of atropine. When acetaldehyde was given into the sinus node artery, positive inotropic and chronotropic effects were dose-relatedly produced from 0.1 to 30 mumol. Acetaldehyde-induced positive inotropic and chronotropic effects were inhibited by treatment with propranolol. However, these positive effects were not suppressed by either imipramine or tetrodotoxin, although imipramine inhibited tyramine-induced actions and tetrodotoxin suppressed acetylcholine-induced positive chronotropic and inotropic effects in atropine-treated preparations. From these results, it is suggested that ethanol has direct depressant properties on pacemaker and contractile activities, and that acetaldehyde has stimulating properties through adrenergic beta-receptors but neither through tyramine-like action nor nerve excitation.