1. The experiments were done on single superfused nodes of Ranvier of frog motor fibres (Rana esculenta) with the double air gap method. The node was stimulated at a rate of 80 impulses/sec and the maximum rate of rise (\(\dot V_A \)) and the duration (D) of the action potentials were recorded. 2. The perfusate was suddenly changed from a Na-poor solution without tetraethylammonium (TEA) to a Na-rich solution containing 1 mM TEA. From the increase in\(\dot V_A \) the time course of [Na]o and, indirectly, of [TEA]o at the membrane was obtained. The latter change was compared with the rate of TEA action as reflected by the increase inD. Onset of the TEA effect was clearly slower than the change of drug concentration while on washout,D responded immediately to the decrease of [TEA]o. 3. Comparable rates of onset and offset of the effect were observed when 1 mM TEA was applied at constant [Na]o. The rate of onset was not increased by applying 5 mM TEA; on washoutD declined more slowly than in the experiments with 1 mM TEA. 4. The comparatively slow rate of onset is tentatively explained by a secondary process. The rate of offset, however, is determined by the rate at which TEA diffuses away (time constant of about 50 msec) implying a rapid dissociation of a TEA complex with a receptor on the outer membrane surface.