Proteins in the inner surface of the squid axon membrane were labeled by intracellular perfusion of [3H]N-ethylmaleimide (NEM), which forms covalent bonds with free sulfhydryl groups. The excitability of the axon was unaffected by the [3H]NEM perfusion. After washout of the unbound label, the perfusate was monitored for the release of labeled proteins. Labeled proteins were released from the inner membrane surface by potassium depolarization of the axon only in the presence of external calcium ions. Replacement of the fluoride ion in the perfusion medium by various anions also caused labeled protein release. The order of effectiveness was SCN- greater than Br- greater than Cl- greater than F-. The extent of labeled protein release by the various anions was correlated with their effects on axonal excitability. The significance of these results is discussed.