The 78,000 × g microsomal fraction of canine ventricular myocardium effected a 20-fold concentration of [³H]norepinephrine from a 10^-9 M solution. The [³H]norepinephrine bound was displaced by physiologic concentrations of the beta-adrenergic catecholamines isoproterenol, epinephrine, and norepinephrine, in that order, which is the order of effectiveness of their actions on the force and rate of cardiac contraction. Alpha-adrenergic compounds did not displace [³H]norepinephrine until concentrations four orders of magnitude greater were reached. The beta-adrenergic blocker propranolol displaced at 10^-6 M, whereas the alpha-adrenergic blocker phentolamine was inactive. Metabolites of the catecholamines did not compete for this binding site. It is concluded that, on the basis of specificity and affinity of binding, these microsomal particles are likely to contain the beta-adrenergic receptor.