In a variety of nerve fibers, cells and other excitable tissues, the electric responses to electric stimuli were found to be accompanied by a transient swelling of the tissues. The rising phase of this swelling coincides with that of the electric response. By use of a heat-sensor made of polyvinylidene fluoride film, it was also shown that the electric responses of many types of excitable tissues are accompanied by simultaneous heat production. To elucidate the origin of this swelling and heat production, the process of Ca2+-Na+ exchange in synthetic anionic gel beads and rods was investigated. It is asserted that the observed signs of nerve excitation are manifestations of a rapid structural change of the cortical gel layer of the protoplasm, plasmalemma-ectoplasm complex. The importance of the rapid movement and rearrangement of water molecules in the cortical gel layer in association with excitation processes is emphasized.