Alteration of ligand binding to dopamine D 2 receptors through activation of adenosine A 2A receptors in rat striatal membranes has been studied by means of kinetic analysis. The binding of dopaminergic agonist [ 3H]quinpirole to rat striatal membranes was characterized by the constants K d=1.50±0.09 nM and B max=115±2 fmol/mg of protein. The kinetic analyses revealed that the binding had at least two consecutive and kinetically distinguishable steps, the fast equilibrium of complex formation between receptor and agonist ( K A=5.9±1.7 nM), followed by a slow isomerization equilibrium ( K i=0.06). Activation of adenosine A 2A receptors by CGS 21680 caused enhancement of the rate [ 3H]quinpirole binding, altering mainly the formation of the receptor-ligand complexes ( K A) as well as the isomerization rate of this complexes ( k i), while the deisomerization rate ( k −i) and the apparent dissociation rate remained unchanged.