Using squid giant axon, an experimental survey was performed on restoration of the membrane excitability which had been partially suppressed. Among reagents examined, a combination of 400 mM KF, 50 micrometers tyrosine, 1 mM ATP, 1 mM Mg ions and 2 micrometers cAMP was found to induce the restoration of the excitability to a large extent. Further addition of a small amount of either porcine brain microtubule proteins or the squid axoplasm was found to support complete restoration. The experiments suggest that tubulin-tyrosine ligase contained in the porcine brain microtubule protein fraction or the squid axoplasm maintains the coupling between cytoskeletal structures and the plasma membrane.