[3H]-Dihydroalprenolol ([3H]-DHA) binds to cerebral membranes of the frog, chick, rat, mouse, rabbit and human with a dissociation equilibrium constant (KD) of about 1 nM and displays binding characteristics indicative of an interaction with beta-adrenoceptors. However, the maximum number of specific binding sites labelled by this beta-adrenoceptor ligand varies substantially between the species with the chick and mouse having the highest, and the frog the lowest density. The structure--activity relationships of adrenergic agents to inhibit specific [3H]-DHA binding suggests that whereas the membrane sites from all the species had similar affinities for non-selective beta-adrenergic agents, several drugs that have been reported to show selectivity for beta1-adrenoceptors demonstrated considerably higher affinities for mammalian rather than avian or amphibian membrane sites. By this pharmacological criteria it is likely that all the beta-adrenoceptor binding sites in frog and chick cerebral tissue have properties resembling beta2-receptors. However, in mammalian cerebral cortex, evidence is presented that beta1- and beta2-adrenoceptors coexist in a ratio of 70%/30% respectively.