It is well known that ion composition significantly affects action of various pharmacological agents. In the present work we studied influence of external sodium on the block of Ca2+ -permeable AMPA receptor channels by dicationic derivative of phenylcyclohexyl IEM-1925. Experiments were performed on native receptors of giant striatal interneurones isolated from the rat brain slices. Registrations were done with the aid of whole-cell patch clamp technique. We found that partial substitution of external sodium by sucrose potentiated the blocking action of IEM-1925. The effect was voltage-dependent, being more pronounced at hyperpolarized voltages. The analysis of kinetics of the IEM-1925 action demonstrated that lowering of external sodium facilitated blocking action, whereas stability of the drug-channel complex remained unaffected. We conclude that the current carrying sodium ions compete with blocking cation IEM-1925 for an intrapore binding site.