Squid giant axons were perfused intracellularly with solutions containing various kinds of proteases (1 mg/ml). Except for a 10 micro layer inside the axolemma the axoplasm was removed by a 5 min perfusion with Bacillus protease, strain N' (BPN'). The resting and action potentials were unchanged and the axon maintained its excitability for more than 4 hr on subsequent enzyme-free perfusion. After perfusion with protease solution for 30 min the axoplasm was almost completely removed. The excitability was maintained, but the action potential became prolonged and rapidly developed a plateau of several hundred milliseconds. The change was not reversible even when the enzyme was removed from the perfusing fluid. Two other enzymes, prozyme and bromelin, also removed the protoplasm without blocking conduction. Trypsin suppressed within 3 min the excitability of the axon. It is suggested that the proteases alter macromolecules in the excitable membrane and thus affect the shape of the action potential.