Abstract β-Adrenergic agonists cause adenylate cyclase activation via a conformational change of their receptor. This was earlier revealed by the use of N-ethylmaleimide as a structural probe: agonist-bound receptors are rapidly inactivated by 0.1 mM N-ethylmaleimide while free and antagonist-bound receptors remain unaffected. Furthermore, the β-adrenergic receptors only change conformation when coupled to the guanine nucleotide regulatory component of the adenylate cyclase system. It is shown in this report that treatment of turkey erythrocyte membranes with the sulphydryl-specific agent 2,2′-dinitro-5,5′-dithiodibenzoic acid and elevated concentrations of N-ethylmaleimide (1–10 mM) do not affect the total receptor number, but prevent the ability of β-adrenergic agonists to mediate conformational changes. This effect is mimicked by GTP. These three compounds cause also a two-to four-fold decrease in agonist affinity. Both phenomena may be explained by the ability of the reagents to prevent, and of GTP to reverse, the functional coupling between the receptors and the regulatory component of the adenylate cyclase system. Removal of Mg 2+ from the incubation medium (i.e. presence of 1 mM EDTA) produces a similar decrease in agonist affinity, but does not impair the ability of agonist/ N-ethylmaleimide to inactivate the receptors. This suggests that Mg 2+ increases the agonist affinity for the receptor-regulatory component complex, but is not required for its formation.