This study demonstrates that the tachycardia produced by systemic injections of the beta-adrenoceptor agonist, isoproterenol (10 microg/kg, i.v.), in conscious rats were not reduced after injection of the selective beta(1)-adrenoceptor antagonist, atenolol (1 mg/kg, i.v.), or after subsequent injection of the beta(1,2)-adrenoceptor antagonist, propranolol (1 mg/kg, i.v.). The hypotensive responses produced by isoproterenol were slightly diminished by atenolol and markedly diminished by propranolol. The tachycardia produced by catecholamines released for cardiac sympathetic nerve terminals were blocked by atenolol. These results suggest that the hypotensive actions of a 10 microg/kg dose of isoproterenol are mediated by activation of beta(1,2)-adrenoceptors whereas the increases in heart rate may be due to activation of another type of beta-adrenoceptor in cardiac pacemaker cells.