We studied the role played by postjunctional adrenoceptors in the vasoconstrictor responses of perfused tail arteries of young prehypertensive (4-6 weeks old) and adult (14-16 weeks) spontaneously hypertensive rats (SHR) with norepinephrine (NE), phenylephrine and epinephrine. Age-matched Wistar normotensive rats (WNR) were also used. In tail artery of young SHR, as well as in age-matched WNR and adult WNR, responsiveness to the vasoconstrictor agents did not differ. Determination of KB values of prazosin using phenylephrine and epinephrine as agonists and analysis of Schild plots indicate that responsiveness to the vasoconstrictor agents in young SHR tail artery is mediated by alpha 1-adrenoceptors. Tail arteries of adult SHR were more responsive to epinephrine than were tail arteries of age-matched WNR. In tail arteries of adult SHR, the KB value of prazosin against epinephrine remains undetermined since the slope of the Schild plot statistically differs from 1.0 (0.32). These results strongly suggest that in perfused tail arteries of adult SHR, responses to epinephrine are also mediated by alpha 2-adrenoceptors. We postulate an important but unknown role for the interaction between epinephrine and postjunctional alpha 2-adrenoceptors in the development and maintenance of genetically-induced hypertension.11473-7