Rapid mechanical and thermal changes in slices of the Torpedo electric organ evoked by electric stimulation were investigated. The organ was found to swell simultaneously with the postsynaptic potential. This swelling was followed by prolonged shrinkage of the organ. These findings may be explained from the proven facts that the acetylcholine(ACh)-receptor proteins have a large binding capacity for Ca-ions and that addition of ACh to the medium can cause release of Ca-ions from the receptor proteins.