1 The a 1 -adrenoceptor antagonist properties of the b-adrenoceptor nonconventional partial agonist, CGP 12177A, was investigated in functional assays in rat aorta and in radioligand binding assays in rat cerebral cortical membranes. In addition, binding affinities of other b-adrenoceptor ligands were measured to investigate any correlation between a 1 -adrenoceptor affinity and relaxant potency in phenylephrine-constricted rings. 2 In functional studies, CGP 12177A produced parallel rightward shifts of the phenylephrine CRC with no reduction in the maximum responses. Schild regression analysis gave a straight line with a slope of 0.95 (95% CL: 0.87-1.04), suggesting reversible competitive antagonism, and gave a pK B value of 5.26. In contrast, CGP 12177A (p300 mM) had no effect on contraction induced by the thromboxane-mimetic, U46619. Abbreviations: B max , total number of receptor binding sites; CL, confidence limits; CRC, concentration-response curve; DMSO, dimethyl sulphoxide; E max , maximum response; IC 50 , concentration (M) of competing ligand that inhibits binding of radioligand by 50%; pEC 50 , negative logarithm of the concentration (M) of relaxant that produces 50% of its maximum response; pK B , negative logarithm of the equilibrium dissociation constant, obtained from functional experiments; pK D , negative logarithm of the equilibrium dissociation constant, obtained from saturation experiments; pK i , negative logarithm of the equilibrium dissociation constant, obtained from competition experiments; s.e.m., standard error of the mean