Biphasic potentials are recorded in the column and tentacles ofPachycerianthus fimbriatusfollowing stimulation by a single electrical shock of a through conducting system anywhere on the column. Magnesium is shown to first increase the conduction delay and number of potentials in the response and then eliminate it. A second electrical response, trains of spikes, stimulated by application ofAnemia“juice” is present in the tentacles.