Earlier, we have shown a strong inhibitory effect of donepezil on K+-current of molluscan neurons (Solntseva et al., Comp Biochem Physiol 144, 319-326, 2007). In the present work, a possible interaction of donepezil with the external mouth of the channel was examined using, as a tool, tetraethylammonium (TEA), a classical antagonist of potassium channels. Experiments were conducted in isolated neurons of snail Helix aspersa using the two-microelectrode voltage-clamp technique. A high-threshold slow-inactivating K+-current involving Ca2+-dependent (I (C)) and Ca2+-independent (I (K)) components was recorded. The I (C) was estimated at 30 mV, and I (K) at 100 mV. The IC(50) values for blocking effect of donepezil on I (C) varied from 5.0 to 8.9 microM in different cells. Corresponding values for I (K) varied from 4.9 to 9.9 microM. The IC(50) values for blocking effect of TEA on I (C) lied in the range of 200 to 910 microM, and on I (K) lied in the range of 100 to 990 microM. The comparison of the effects of donepezil and TEA on the same cells revealed significant correlation between IC(50) values of these effects. The value of Spearman coefficient of correlation (r) was 0.77 for I (C) (P < 0.05), and 0.82 for I (K) (P < 0.05). In the presence of TEA, the effect of donepezil, both on I (C) and I (K), appears significantly weaker than in control solution. Dose-response curves of donepezil effect both on I (C) and I (K) were shifted right along horizontal axis when donepezil was applied in combination with TEA. Results suggest that TEA interferes with donepezil and precludes the occupation by donepezil of its own site. We suppose that the site for donepezil is situated near the TEA site with possible overlap.