Abstract
Optical and mechano-electric detectors show that the squid giant axon swells when an action potential is generated. The maximum swelling is reached at the peak of the action potential. The undershoot of the membrane potential is associated with a marked shrinkage of the axon. These mechanical changes also are demonstrated in axons from which a major portion of the axoplasm has been removed. The effects of changing the tonicity of the external medium and of applying several chemical reagents are studied.