In electrophysiological investigations, relative permeabilities of somatic neuronal membrane in the winkle for K+, Rb+ and Tl+ have been obtained which made it possible to establish the selectivity row for this membrane. This row in principle coincides with similar rows for the membranes of other excitable formations. The difference lies in higher permeability of the membrane investigated for sodium. Permeability coefficients for K+, Rb+, Tl+ and Na+ in this membrane were also determined. In marine molluscs, sodium and potassium permeabilities of the membranes are lower than in fresh-water or terrestrial ones.