The effects of butropium on chronotropic and inotropic activity were investigated in 10 isolated atrial preparations which were suspended in a bath and perfused with arterial blood led from a carotid artery of heparinized support dog. Butropium injected into the cannulated sinus node artery in a dose range of 3 to 100 microgram produced a dose-dependent positive chronotropic and inotropic effect. At relatively higher doses above 30 microgram, slight and brief initial negative chronotropic and inotropic effect followed by a long-lasting positive effect was observed. The positive effect of butropium was markedly suppressed by propranolol or carteolol, but it was not affected by tetrodotoxin or desmethylimipramine. From these results, a large amount of butropium may induce positive chronotropic and inotropic effect through cholinergic blockade and in part stimulation of adrenergic beta-receptors.