[en] Binding of [³H]-dihydroalprenolol ([³H]-DHA) to rat cardiac membranes was rapid and reversible (k₁ = 0.633 to 0.701 x 10⁶ M^-1s^-1 and ksub(-1) = 0.0017 to 0.0043 s^-1). [³H]-DHA bound to a single class of binding sites with an equilibrium dissociation constant (Ksub(d25⁰C) of 5.7 +- 1.1 x 10^-9M. This binding was specific and the order of potency of adrenoceptor agonists in competing for the binding sites was (-)-isoproterenol > (+-)-isoproterenol >(+)-isoproterenol > (-)-adrenaline > (-)-noradrenaline. This was in agreement with the β₁ nature of the cardiac β-receptors. Cardioselective β-blockers (i.e. metoprolol, acebutolol and practolol) were shown to have lower binding site affinities, when compared to other blockers. This may be related to steric hindrance by the side-chain at the aromatic end of these molecules. (author)