Abstract 【Aim】 A Glu-Asn-Phe (ENF) peptide, paralytic peptide (PP) identified in several lepidopteran hemolymph induces rapid and rigid paralysis defined by a tonic contraction when injected into larvae. This study aims to determine the optimal amount and maximum contraction that PP induces in live larvae of the silkworm, Bombyx mori, and to examine the possible change of other physiological index, such as pulse velocity and ion concentration which might accompany the contraction. 【Methods】 The intensity of body contraction and pulse velocity were monitored after injection of various amounts of PP into the body of the 5th instar larvae. And the ion concentrations in hemolymph, fat body and digestive tube were measured by atomic absorption spectroscopy. 【Results】 PP at the concentration of 50 ng/g animal triggered the most potent contraction with no lethal effect. The maximum body contraction was seen between 4 and 5 min after injection. Digestive tube distortion was found to accompany body wall contraction, and pulse velocity decreased when the body reached the maximum contraction. In addition, extracellular Ca2+ was required for the contraction and PP also stimulated a sharp decrease then a slow recovery of Cl- concentration in hemolymph. 【Conclusion】 The paralytic activity of PP not only affects the body wall, but also digestive tube and dorsal vessel of silkworm larvae, and is associated with disruption of Cl- homeostasis. Our results will provide reference for studying the physiological role of PP in an animal model.