The impedance variation cycle (IVC) of axonal membranes has been described in terms of a dephosphorylation-phosphorylation cycle. It has been experimentally approached by preparing membranes from the walking nerves of crabs and from the main electric organ of Electrophorus electricus . Membrane proteins were solubilized in 1% Lubrol-PX and purified by chromatographic techniques. We have obtained a protein fraction which was phosphorylated by low [?-(32)P]-ATP concentrations, ranging from 5 x 10(?8)M to 10(?7)M. This fraction of high molecular weight proteins has been partly characterized in denaturing conditions revealing the presence of several protein species with different specific phosphorylation activity. At this experimental stage the two types of investigated materials displayed distinct patterns of phosphorylation versus molecular weight. The influence of tetrodotoxin and veratridine, on the phosphorylation process is a rather complex inhibitory effect, now under further investigation.