The ability of the beta 1 partial agonists prenalterol and corwin , to displace the binding of [3H]dihydroalprenolol to the heterogeneous populations of beta-adrenoceptors on rat and rabbit lung membranes was compared. Both drugs exhibited higher affinity for the binding sites in rabbit lung (predominant beta 1) as compared to rat lung (predominant beta 2). Curve fitting analysis of the displacement curves into high affinity (beta 1) and low affinity (beta 2) components indicates that both drugs exhibit beta 1 selective affinity. Further competition experiments performed in the presence of carefully calculated concentrations of highly selective beta 1 (atenolol) or beta 2 (ICI 118,551) antagonists to produce homogeneous receptor subtype populations in rat and rabbit lung membranes respectively, confirmed this beta 1 selective affinity. These results suggest an approximate selective affinity ratio (beta 1 to beta 2) of ten and forty fold for prenalterol and corwin respectively.