The distribution of alpha 1-adrenoceptor and its subtypes in isolated aortae was compared between 12 month- and 10 week-old Wistar rats by determinating vasoconstrictor responses. In the 12 month-old rats, compared with the 10 week-old rats, (1) the maximal contraction induced by norepinephrine (NE) was reduced, without significant alteration of pD2 value; (2) the inhibitory effect of chlorethylclonidine (an irreversible antagonist for alpha 1B and alpha 1D subtype) on NE-induced contraction was weaker; (3) using NE as an agonist, the pA2 value for WB4101 (an alpha 1A- and alpha 1D-selective antagonist) was not changed, but the pA2 value for BMY7378 (an alpha 1D-selective antagonist) was decreased, while the pA2 value for sertindole (an alpha 1A-selective antagonist) was increased. Thus, differently from the 10 week-old rats in which only alpha 1D-adrenoceptors mediate NE-induced contraction in aortae, both alpha 1A- and alpha 1D-adrenoceptors (mainly alpha 1A) mediate the response in the 12 month-old rats.