1. 1. Acetone powder from the anemone Condylactis gigantea contains a single component which paralyzes crustacea. 2. 2. This same component transforms action potentials in crustacean neurons into prolonged plateau potentials of up to several seconds' duration. 3. 3. The conduction block which eventually occurs is not due solely to depolarization. 4. 4. It is suggested that the plateaus are caused in part by a prolonged membrane permeability after initial excitation.