alpha 1-Adrenoceptors in rat brain membranes, labeled by the antagonist [3H] prazosin, increase their affinity for clonidine several-fold when sodium ions are added. The conversion of alpha 1-adrenoceptors, buy sodium ion, into a conformation, which has higher affinity for the agonist clonidine, indicates that these receptors as the alpha 2-adrenoceptors are modulated by cations in vitro.