Abstract: Distribution of 5-HT,GnRH and their receptors within ovaries of Crassostrea gigas was studied using the immunocytochemistry and double staining technique.The results revealed that 1)5-HT and GnRH receptors existed in the oocytes of different development stages of the ovary;2)the reactive substances of 5-HT and GnRH receptors were distributed along the membrane of oocytes and showed dark brown,but nucleus showed immunonegative reaction;3)5-HT,GnRH and their receptors co-existed in the cytoplasm and membrane of oocytes of different development stages.The above results indicate that the oocytes of Crassostrea gigas can not only produce 5-HT and GnRH,but also express their receptors.5-HT and GnRH may exert regulation action on the oocytes by autosecretion.