Summary: A great deal of experimental observations at α-adrenoceptors may be explained if two points are considered: (1) the α1-/α2-adrenoceptor ratio (selectivity ratio) of a given drug and (2) the ratio of α1-/α2-adrenoceptors in the target organ or tissue. The α1-/α2-selectivity ratio tor 10 agonists was determined in pithed rats by their hypertensive effect in the presence or absence of the antagonists rauwolscine or prazosin, respectively. Treatment with a β-blocking agent made it possible to test drugs with additional β-adrenergie potencies (phenylethylamines). Methoxamine exerted the highest α1-/α2-selectivity ratio, the azepine derivative B-HT 920 the lowest ratio. i.e.. the highest selectivity for α2-adrenoceptors. Using the two selective agonists methoxamine and B-HT 920. respectively, the α1-/α2-adrenoceptor ratio (importante) in several target systems was estimated (determination of equieffec-tive doses). Results indicated the prevalence of α1-adrenoceptors in the nic-titating membrane. and the equal importante of both subtypes at peripheral postsynaptic vascular sites in rats. as well as in cats. At presynaptic sites (cardiac nerve. rats), α2-adrenoeeptois are prevalent. A comparison of equief-fective doses of B-HT 920 and clonidine in cats indicated the α2-nature of the central adrenoceptors which mediate hypotension.