3 The novel ax-adrenoceptor antagonist RS 17053 had a relatively high affinity for the aAadrenoceptors mediating contraction of the rat epididymal vas deferens (pA2 9.5) compared with the alB-adrenoceptors in the rat spleen (pA2 7.2) or the aID-adrenoceptors in the rat aorta (pKB 7.1), in agreement with its selectivity for the expressed a,.-clone. However, RS 17053 had over 100 fold lower affinity for the aIA-adrenoceptors mediating contraction of the rat portal vein (pKB 7.1) and human prostate (pKB 7.1) compared with its affinity for the alA-adrenoceptors in the rat epididymal vas deferens or the expressed aa-clone. 4 The difference in affinity of RS 17053 between the rat epididymal vas deferens and rat portal vein cannot be explained by a species difference in the receptor. Therefore RS 17053 may distinguish between subtypes of the aIA-adrenoceptor in the rat portal vein and human prostate compared with those in the rat epididymal vas deferens or the expressed ala-clone.