Tetanic electric stimulation of Helix foot evokes sensitization of escape reaction. This behavioral sensitization and posttetanic potentiation (PTP) of acetylcholine-induced inward current (ACh-current) in command Helix neurons of escape behavior were similar. Antagonist of serotonin receptors methiothepin prevents the PTP of the ACh-current and behavioral sensitization. Serotonin disrupts the PTP of the ACh-current. It is suggested that the increase in cholinosensitivity of the command neurons with the involvement of methiothepin-sensitive serotonin receptors may be the cellular postsynaptic mechanism of behavioral sensitization of Helix escape reaction.