An electron microscopic study was made on squid giant axons perfused with a kind of protease, BPN' ( Bacillus protease, strain N'). The axon treated with this enzyme maintained excitability for more than 1 hour. After 8 min. of perfusion, practically no axoplasm remained inside the plasma membrane. The protease affected the internal structure of the Schwann cell to some extent, whereas the structure of the plasma membranes appeared to be normal at the level of the electron microscope. These axons can be used for physiological investigation of the fundamental mechanism of excitability.