The interaction of the alpha 2-adrenoceptor agonists clonidine and xylazine with the alpha 1-adrenoceptors in the rat and rabbit aortas was investigated. In preparations preconstricted with phenylephrine (10(-6) M), cumulative addition of clonidine and xylazine induced concentration-related vasodilatation in the presence of the alpha 2-adrenoceptor antagonist yohimbine and the beta-blocker propranolol. Neither clonidine nor xylazine treatment inhibited 5-HT- and KCl-induced vasoconstriction. It is concluded that alpha 2-agonists have affinity for alpha 1-adrenoceptors in rat and rabbit aortas.