The optic nerve of Necturus has proved a useful preparation for the study of glial cell membranes in vivo and in vitro with anatomical relations to axons intact and isolated following axon degeneration. The glial membrane potential behaves as a selective potassium diffusion potential; there is no evidence of a significant permeability to other naturally occurring ions. The specific membrane resistance of the glial cells is high compared to that of neurones; there are low-resistance intercellular connexions among the cells which permit the passage of both ions and the dye Lucifer Yellow. The cells are readily and reversibly uncoupled by procedures which decrease the intracellular pH. There is no evidence for voltage-sensitive sodium channels in the membrane. Following sodium gain and potassium loss the membrane displays a potassium-dependent strophanthidin-sensitive electrogenic sodium pump. The glial membrane is depolarized by potassium released from active axons as well as by glutamate. The glial depolarization contributes to potentials recorded with surface electrodes. Depolarization by K+ plays a role in the redistribution of K+ which locally accumulates around active neurones and also affects glial metabolism and glucose uptake.