The cardiovascular activity of TPI in lowering the blood pressure and heart rate in anesthetized rats was studied using pharmacological and physiological techniques. The hypotensive and bradycardiac actions induced by a bolus intravenous injection were significantly inhibited by pretreatment with atropine (0.1 mg/kg; i.v.). Vagotomy almost abolished the hypotensive response but moderate bradycardia still remained. Microinjection of TPI (1-10 nmol) into the nucleus tractus solitarii of brain stem produced a prominent dose related hypotensive and bradycardic effects. TPI also produced a decrease in the force of contraction of the isolated right atrium of guinea pig. These results suggest that the effects of TPI may involve an activation of the vagus nerve and suppressive action on myocardial tissue.