Rhythmic electrical stimulation of the snail foot leads to sensitization of the defensive reflex. This sensitization has dynamics similar to those of posttetanic potentiation of the amplitude of the acetylcholine-evoked influx current of defensive behavior command neurons in the common snail. It is likely that an increase in the cholinosensitivity of the somatic membrane of defensive behavior command neurons in the common snail may be involved in the mechanism of sensitization of the animal's defensive response. Methiothepin, an antagonist of serotonin receptors, prevented the posttetanic potentiation of the acetylcholine-evoked influx current as well as behavioral sensitization. Serotonin, like methiothepin, also impaired posttetanic potentiation of the acetylcholine-evoked influx current. It is suggested that methiothepin-sensitive serotonin receptors are involved in the postsynaptic mechanism of behavioral sensitization.