Constant pressure perfusion of the canine sinus node artery was performed in 9 isolated atrium preparations and in 4 in situ preparations anesthetized with i. v. sodium pentobarbital, 30 mg/kg. Insulin, 1, 3, 10 or 20 units, was injected into the sinus node artery. Doses of more than 10 units of insulin induced a positive chronotropic response in the isolated preparations and in 2 out of 4 in situ preparations. The positive chronotropic effect became less prominent by repeated injections of insulin but never disappeared completely. This insulin-induced sinus acceleration was not completely blocked by propranolol which blocked norepinephrine action. It is concluded that insulin at large dose level induces a positive chronotropic response which is not due to cardiac β-adrenergic stimulation.