Hemodynamic mechanisms of Co(2+)-evoked hypotension were studied in pentobarbital--anesthetised male Wistar rats. Bolus i.v. administration of cobalt sulphate (30, 90 and 270 mg/kg of Co) evoked dose-dependent hypotension followed by tachycardia and increase in left ventricular contractility index (dP/dt/P). Administration of beta-adrenoblocker propranolol (2 mg/kg) attenuated Co evoked (270 ug/kg) tachycardia and unmasked its cardiodepressive effects (dP/dt/P decreased by 11% and end-diastolic pressure increased by 55%). Cardiodepressive Co2+ effects were prevented by administration of verapamil (0.4 mg/kg). In the second group hemodynamic mechanisms of Co-evoked hypotension were studied with radioactive microspheres. Co2+ infusion (55 mg/kg) lowered blood pressure (in average by -11%) by decreasing total peripheral resistance (-25%). Cardiotoxic effects were obtained with a ++larger dose 80 mg/kg/min. It is concluded that Co decreased blood pressure by vasodilatator action and its cardiotoxic effects attenuated by sympathetic counterregulation and may be prevented by verapamil.