The effects of imipramine were investigated on the blood pressure and heart rate in the intact dog and on the atrial rate and contractile force in the isolated atrial muscle perfused with arterial blood of the donor dog. A continuous infusion of small doses of imipramine (30 micrograms/Kg/min, i.v., 30 min) produced an increase in the blood pressure and heart rate of the donor dog and the positive chronotropic and inotropic effects of isolated atria. These responses were blocked by treatment with propranolol. When a large dose of imipramine (1 mg/Kg/min, i.v., 15 min) was administered to the donor dog, the blood pressure fell and the heart rate initially increased and then decreased (i.e., 15 min later it decreased approximately 20% under the control level), and in isolated atria and the developed tension initially increased and 15 min later decreased but the atrial rate maintained over the control level. Examined doses of imipramine caused a potentiation of norepinephrine-induced action, and a large dose of imipramine significantly diminished norepinephrine-induced reflex bradycardia and/or frequently inverted to tachycardia. Moreover acetylcholine-induced reflex tachycardia was suppressed by imipramine treatment. From these results, it is concluded that imipramine may cause a hypotension mainly due to peripheral vasodilation, and may induce a suppression of baroceptive reflex mechanism.