Abstract: The distribution of 5-HT in main cells of both ovary and hepatopancreas at different ovary stages of Neomysis japonica was observed and compared by using immunohistochemical method.These cells included follicule cell,oogonium,endogenous vitellogenic oocyte,exogenous vitellogenic oocyte,mature oocyte,embryonic cells,blister like cells,resorptive cells,fibrillar cells.The results showed that changes on the distribution of positive 5-HT cells were observed with ovary development.All the follicule cells are stained positively at each ovary stage.As for the oocyte,both the cytoplasm and nucleus are stained positively during the early developmental stages,including Stage Ⅰ,Ⅱ,and Ⅲ.But the positive intensity presents a negative trend with ovary development.The oocytes are stained negatively at Stage Ⅳ whereas they were found weak positive in Stage Ⅴ.In contrast,the hepatopancreas cells are stained positively during each stage.Moreover,there are no significant differences of positive 5-HT in different types of hepotapancreas cells during the ovary development.In addition,the 5-HT positive intensities of oocyte were consistent with hepatopancreas cytoplasm throughout all ovary developmental stages(except Stage Ⅳ).