The mechanism of nerve excitation was studied from a macromolecular viewpoint. During excitation of a crab nerve and a squid giant axon, the membrane surface was found to exhibit a small, rapid outwards movement. Simultaneously, the pressure detected with a detector pressing the membrane surface was shown to rise rapidly. These newly discovered phenomena are regarded as a sign of swelling of the axolemmaectoplasm complex during the action potential. By using pyroelectric polymer material, polyvinylidene fluoride, faithful records of temperature changes associated with excitation of a crab nerve were obtained.