1. Superior cervical ganglia of adult rats were excised and maintained in vitro in stable conditions. Potentials were recorded with external electrodes. After transmission was blocked by mecamylamine, a small potential change was recorded from the rostral area of the ganglion in response to preganglionic stimulation.2. This electrical response was identified as the presynaptic action potential recorded from the nerve terminals by a number of criteria based on histological and physiological considerations including the disappearance of the spike in a glucose free solution. As shown by Nicolescu, Dolivo, Rouiller & Foroglou-Kerameus (1966) on the same preparation this condition causes an irreversible and selective lesion of the presynaptic nerve endings.3. A suitable concentration of mecamylamine permitted the presynaptic response and the excitatory post-synaptic potential (EPSP) to be recorded simultaneously. As the stimulus was increased, the EPSP increased linearly with the amplitude of the presynaptic response.4. After replacement of potassium ions in the bathing solution by caesium and during the early phase of post-tetanic facilitation there was an increase in the presynaptic response accompanied by a disproportionate increase in the EPSP.5. No changes in the presynaptic response were found in the presence of the following drugs, all of which depressed the EPSP: acetylcholine, hemicholinium, curare, further doses of ganglion-blocking agents, and high Mg(2+) and low Ca(2+) concentrations.6. Ouabain (4.5 x 10(-4)M) reversibly decreased the amplitude of the presynaptic response and increased the spontaneous release of transmitter. The EPSP was at first enhanced and then depressed.