Ciguatera poisoning is the most common fish food poisoning encountered in man. Electrophysiological studies were performed on the ventral nerve of the tail on adult rats following intraperitoneal injection of toxic fish extract. Significant slowing of both mixed and motor nerve conduction velocities and F wave responses were recorded. Motor and mixed nerve amplitudes were significantly reduced. Both absolute and supernormal periods were significantly prolonged together with an exaggeration of the supernormal response. These findings indirectly suggest that ciguatoxin acts on mammalian nerve by prolonging sodium channel activation.