The effects of a treatment with reserpine on alpha-adrenoceptor-mediated contractile responses of rat tail arteries were investigated in vitro. The potency of norepinephrine was slightly increased in arteries obtained from rats treated with reserpine. There was no significant change in the sensitivity of the arteries to serotonin, KCl and selective alpha 1-adrenoceptor agonists (methoxamine and phenylephrine). However, the potency of clonidine and UK-14,304, both selective alpha 2-adrenoceptor agonists, was greatly increased. UK-14,304-induced contractions of the arteries from rats treated with reserpine were inhibited strongly by rauwolscine, a selective alpha 2-adrenoceptor antagonist, but only slightly by corynanthine, a selective alpha 1-adrenoceptor antagonist. The contractions caused by re-introduction of Ca2+ during exposure to UK-14,304 but not to methoxamine in a Ca2(+)-free medium were potentiated by treatment with reserpine. Bay K 8644, an agonist of Ca2+ channels, produced a concentration-dependent contraction only in the arteries from rats treated with reserpine. These results suggest that treatment with reserpine potentiates alpha 2- but not alpha 1-adrenoceptor-mediated responses in rat tail arteries and that the potentiation could be related to changes in mechanisms linked to Ca2+ influx.