[3H](±)Carazolol, a newly available β-adrenergic receptor antagonist, has been used to characterize β-adrenergic receptor subtypes present in membrane vesicles derived from canine ventricular myocardium and canine lung. [3H](±)Carazolol binding is saturable, of high affinity, and in displaceable by β-adrenergic agents in accordance with their known pharmacological potencies. The interaction of carazolol with β-adrenergic receptors is stereospeclflc; the (−) stereoisomer demonstrates greater potency than the (+) stereoisomer. Kinetic analysis of [JH}(±)carazolol interaction with β-adrenergic receptors suggests the presence of two phases of interaction, consistent with initial rapidly reversible “low” affinity association of ligand with receptor, followed by Isomerization to form a high affinity, slowly reversible complex. Through use of a [3H](±)carazolol binding assay based on the high affinity complex, pharmacological specificities of £-adrenergic receptor populations of canine myocardium and lung were quantified. Analysis using computer-assisted techniques suggests a β1/β2receptor ratio of approximately 85%/15% for canine myocardium and 5%/95% for canine lung. In the absence of added guanine nucleotides, comparison of potencies of β-adrenergic agonists in the two membrane systems suggests significant β, selectivity of l-isoproterenol and l-eplnephrine, and nonselectivity of norepinephrine. In the presence of saturating levels of guanine nucleotides, comparison of agonist potencies confirms the non-selectivity of l-isoproterenol and l-epinephrine, and β1selectivity of norepinephrine. These results demonstrate that the state of guanine nucleotide regulation of receptors should be defined when examining agonist selectivities for β-adrenergic receptor subtypes in vitro.