1 The a 1 -adrenoceptor population mediating contraction of caudal artery of rat has been characterized by using quantitative receptor pharmacology. estimates of 9.2, 9.3, 11.2, 9.0, 6.3, 8.7 and 10.0 for RS-17053, SNAP 5089, tamsulosin, 5-methylurapidil, BMY 7378, HV 723 and REC 15/2739, respectively, although deviations from unit slope (possibly re¯ecting a secondary involvement of another a 1 -adrenoceptor) hindered estimations of pK b for some antagonists. The antagonist anity pro®le obtained re¯ects best that described for the a 1A -adrenoceptor. 5 In conclusion, caudal artery of rat contracts in response to NA via activation of at least two a 1 -adrenoceptor subtypes. One of these subtypes displays the pharmacology of the a 1A -adrenoceptor, while the other remains to be de®ned. Use of the novel selective agonist, A-61603, allows for limited pharmacological isolation of the a 1A -adrenoceptor permitting characterization of the properties of selective antagonists.