Visible structural changes following or acconipaiiying electrical stimulation have been studied in a variety of cells [Heilhrniin, '43 ; Tobias and Solomon, '50). In nerve, however, nlthough the more revealing studies of electrical phenomena in excilatiori bespeak cliaiiges in permeability, there have beeit i*elativcly fern direct observations tending csplicity to i*evcal tlic underlying structural cliaracter of such n changc. Eethc ('03, '08. '20) and others ( w e later) hare clescribed size rliaiigw aiid different staiiiirig properties at aiiodally arid cathodally polarized regions in frog nerve, the latter probably due to local changes in pH. von Rluralt ('46) has described liberatioil of thianiiiie from some hound precursor, perhaps connotiiig a kind of structnral change, in frog nerve acconipanying excitation and propagation, Liithy ( '48) has 1 eported a small, delayed iiicrease in ultraviolet absorption by stimulated, siiiglc frog fihers. Flaig ('47) has described ail increase i r i squid giant axon asoplasin viscosity a s a eonsequence of excitation, and F d o r o w ( ' 3 5 ) has reported visihlc changes in frog auricle ganglion cells and cnd feet iipoii sf irnulatioii of pre-ganglionic vagal fibers. A p a ~t from these observations and the failure of Scbmitt and Rclimitt ('40) to detect aiiy change in squid axon biref ringcnce upon stimnlation, there has been. until recently, little mhicli attempts t o describe some structural moiety which, it would seem, must lThis work has been aided by a grant from thc Dr. Wnllacr C. and Clara 4 . Ablwtt Meiiiorial Fnnd of the Univrrsity of Clliertgo.