1. Direct radioligand—binding techniques have been used to characterize and quantify adrenoreceptors in human peripheral lung tissue removed at thoracotomy from ten patients, nine of whom had evidence of obstructive airways disease.
2. [3H]Dihydroalprenolol was used to characterize β–adrenoreceptor sites and [3H]prazosin to identify α–adrenoreceptor sites. Binding of both ligands showed saturability, high affinity, rapid kinetics, reversibility and stereospecificity. The rank order of agonists and antagonists inhibiting specific binding correlated well with known physiological potencies. Specificity of [3H]dihydroal-prenolol binding suggested that the population of lung β–adrenoreceptors is predominantly of the β2 subtype.