The membrane potentials and resistances of L cells were measured using a standard electrophysiological technique. The values obtained in physiological media were around--15 mV and 37Momega, respectively. Almost all the large nondividing L cells (giant L cells) showed spontaneous oscillations of the membrane potential between around -15 and -40 mV. Application of an appropriate electrical or mechanical stimulus was also capable of eliciting responses but such were usually induced only once. The total membrane conductance increased significantly and in parallel with such a hyperpolarizing response. Cooling of the cells and application of metabolic inhibitors to the cells completely blocked the spontaneous oscillation despite the fact that the electrically induced hyperpolarizing response remained. Intracellular K+, Na+ and Cl- concentrations were measured by means of a flame photometer and a chloridometer, and the equilibrium potential for each ion was estimated.