Electrical stimulation of the frog sciatic nerve was found to produce rapid, transient swelling of the 8th and 9th dorsal root ganglia followed by prolonged swelling of the spinal cord. Swelling of the ganglion is analogous to the rapid mechanical change observed in invertebrate axons during excitation. The mechanical change observed in the spinal cord is probably related to prolonged depolarization of the primary afferent fibers near their terminals.