Hemodynamic responses to the selective stimulation of alpha-1 and alpha-2 adrenoceptors were examined in chronically instrumented, conscious dogs. Norepinephrine (0.02-0.1 micrograms/kg/min), a mixed alpha-1/alpha-2 adrenoceptor agonist, phenylephrine (0.2-1.0 micrograms/kg/min), a selective alpha-adrenoceptor agonist and B-HT 920 (0.5-2.0 micrograms/kg/min), a selective alpha-2 adrenoceptor agonist, were infused i.v. after ganglionic (hexamethonium, 30 mg/kg i.v.), beta adrenoceptor (propranolol, 1, mg/kg i.v.) and muscarinic receptor (atropine methylbromide, 0.1 mg/kg i.v.) antagonism. Each of the alpha adrenoceptor agonists increased mean arterial pressure and total peripheral resistance but had no significant effect on cardiac output, stroke volume or heart rate. Equipressor doses of the alpha adrenoceptor agonists caused similar increases in left ventricular systolic and end-diastolic pressure, but there were no significant changes in left ventricular dP/dt or heart rate with any of the alpha adrenoceptor agonists. Selective antagonism of alpha-1 adrenoceptors with prazosin (1 mg/kg i.v.) abolished the pressor and vasoconstrictor responses to phenylephrine but had a lesser effect on the response to B-HT 920. Antagonism of alpha-2 adrenoceptors with rauwolscine (0.1 mg/kg i.v.) caused a significantly greater attenuation of the pressor and vasoconstrictor responses to B-HT 920 than to phenylephrine. The responses to norepinephrine were significantly attenuated by antagonism of either alpha-1 or alpha-2 adrenoceptors. Thus, in the conscious dog with reflex pathways blocked, selective stimulation of either postsynaptic alpha-1 or alpha-2 adrenoceptors increases arterial pressure and total peripheral resistance but does not significantly change heart rate, left ventricular dP/dt, stroke volume or cardiac output.