Vascular smooth muscle alpha adrenoceptors in the proximal end of the rat isolated tail artery have been classified by determining pA2 values and -log KB values for the antagonists prazosin, corynanthine and idazoxan (RX 781094, a new synthetic alpha-2 adrenoceptor antagonist) against norepinephrine. The effects of the antagonists on responses to intramural sympathetic nerve stimulation were also assessed. Artery segments were perfused and superfused with Krebs' solution containing cocaine (4 microM) and propranolol (1 microM). Maximum responses to KCl, norepinephrine and nerve stimulation were not significantly different from one another. Corynanthine (0.1-100 microM), prazosin (10 nM-1 microM) and idazoxan (1-100 microM) caused competitive antagonism of norepinephrine responses with pA2 values consistent with the presence of alpha-1 adrenoceptors. However, idazoxan (10-100 nM) also caused parallel shifts in the concentration-response curves to norepinephrine with -log KB values higher than those consistent with the presence of alpha-1 adrenoceptors. The results have been interpreted to suggest a predominance of smooth muscle alpha-1 adrenoceptors in addition to a subpopulation of smooth muscle alpha-2 adrenoceptors which also contribute to vasoconstrictor responses to norepinephrine. In contrast to the results obtained with exogenous norepinephrine, responses to electrical stimulation were exquisitely sensitive to blockade by prazosin but resistant to idazoxan , suggesting an involvement of an alpha-1 adrenoceptor in these responses. It is concluded that idazoxan may be used to distinguish alpha-1 and alpha-2 adrenoceptors on vascular smooth muscle in vitro and that the results favor the existence of alpha-1 and alpha-2 adrenoceptors in terms of the existing subclassification scheme for alpha adrenoceptor subtypes.(ABSTRACT TRUNCATED AT 250 WORDS)