Summary: Effects of cations and nucleotides on the in vitro binding properties of α-adrenoceptors in rat cerebral cortex membranes are described. Na+ ion converts a large fraction α2-adrenoceptors in the absence or presence of Mg2+ into a low affinity state. The effects of 10 mM Na can be observed in 250 mM Tris-HCL buffer. It is suggested that α2-adrenoceptor agonists alter the affinity of Na+ for a receptor-associated membrane component. α2-Adrenoceptors are linked to guanylnucleotide binding proteins, which may he involved in signal transfer. These sites are altered in their nucleotide binding properties by pre-treatment of the membranes with a medium that allows for ADP ribosylation or and phosphorylation. α2-Adrenoceptors were probed with a new, high affinity ligand, 123J-HEAT. 123J-HEAT binds (30°C) with KD values between 6 and 8 pM to α1-adrenoceptors if Na is present. Na slows down the dissociation of 123J-HEAT in comparison with Mg2. Na as well as Mg2 reverse the inhibitory action of phosphatidic acid on the α1-adrenoceptors labeled by 123J-HEAT. Data on the target size, obtained by radiation inactivation, of the α1-adrenoceptors are presented.