By extending the discovery of the phenomenon of swelling in invertebrate nerve fibers during excitation, we found that the spinal ganglion of the frog swells when the ganglion cells are excited by way of the dorsal root or the sciatic nerve. The time-course of the swelling is similar to that of the action potential of the ganglion cells and is considered to reflect a drastic change in the distribution of water molecules in and near the neuronal membrane. We found also that the spinal cord of the toad and of the frog swells (namely, expands laterally) and retracts (namely, shortens longitudinally) when a volley of impulses enters the cord via dorsal roots. The duration of this mechanical change is very long (about 100 msec or more) and is comparable to that of the dorsal root potential. A volley of impulses entering the cord antidromically via ventral roots produces only a small, brief contraction of the cord. We have examined the basic properties of these mechanical changes in the nervous system and interpreted the results obtained on the basis of our knowledge about the phenomenon of swelling of invertebrate nerve fibers.