The X-organ sinus gland is a major peptidergic neurosecretory system in crustacean, analogous to the vertebrate hypothalamoneurohypophyseal system. Chinese mittenhanded crab (Eriocheir sinensis) X-organ is located on the ventral surface of terminalia in the eyestalk, named MTXO. Three types of neurosecretory cell dissociated from MTXO had been identified on the basis of shape, size and outgrowth fashion. The cell type A and cell type B were immunoactive to antiCHH and antiMIH serum respectively. In the present paper, the wholecell patch clamp technique was used to examine the effect s of 5HT on the excitability and neurosecretion of MTXO cells. The epolariza tion was evoked from the cell type A and cell type B and hyperpolarization from cell type C in response to the rapid application of 5HT (50 μmol). The different frequency overshotting action potential could be recorded after depolarization in the cell type A and cell type B when the 5-HT had been washe d out. Under voltage clamp mode, the increase of cell capacitance was observe d in the cell type A and type B in response to the application of 5-HT. This su ggests that the excitability and releases of CHH and MIH could be induced by 5-HT in the cell type A and type B.