The mechanism of modulation of [3H]raclopride binding to dopaminergic receptors in rat brain striatal membranes by sodium ions was studied by means of equilibrium and kinetic measurements. Among different mono- and divalent cations studied, only sodium and lithium ions significantly enhanced [3H]raclopride binding to rat striatal membranes, but the effect of lithium was considerably smaller if compared with that of sodium. The equilibrium binding studies revealed that the increase in Na+ concentration from 0.5 to 150 mM increased both the radioligand affinity and the number of binding sites. The meaning of these changes was established by kinetic studies, which yielded hyperbolic plots of [3H]raclopride binding rate constants over the radioligand concentration. These plots correspond to the two-step ligand binding reaction mechanism, involving fast binding equilibrium followed by a slow isomerization of the receptor-antagonist complex. Sodium ions did not influence the antagonist affinity for the receptor sites in the first step of the binding process, nor the rate of isomerization of the receptor-ligand complex, but slowed down the rate of deisomerization. This led to a change in the value of the receptor-ligand dissociation constant Kd determined under equilibrium conditions. The same change in deisomerization rate was also sufficient to alter the receptor density (Bmax), measured by the conventional ligand binding procedure.