Abstract: 【Objective】This study aimed to analyze the influence of exogenous 5-hydroxytryptamine(5-HT) on oxidative stress levels and survival rates of Onchidium reevesii under hypoxic stress,and to preliminarily explore the regulatory role of 5-HT in the response of O. reevesii to hypoxic stress. 【Method】Immunohistochemistry and real-time fluorescence quantitative PCR were used to detect 5-HT content and 5-hydroxytryptamine receptors gene(5-HTR) relative expression in O. reevesii. Enzyme-linked immunosorbent assay(ELISA) and real-time fluorescence quantitative PCR were employed to investigate 5-HT content and 5-HTR gene relative expression in different tissues of O. reevesii. Then,comparing the survival rates of O. reevesii under hypoxic stress for 0, 3, 6, 9, 12 and 24 h. The activity of enzymes related to oxidative stress and expression differences of O. reevesii under hypoxic stress were studied by the injection of 5-HT and phosphate buffer saline(PBS). 【Result】5-HT and its receptor were found to distribute in the hepatopancreas,stomach,gonads,intestines,abdominal foot and muscles of O. reevesii. The relative expression levels of 5-HTR gene in the tissues also changed.Tissues showing relatively high expression levels changed from the abdominal foot, intestines and stomach to the gonads as the hypoxic stress time increased. Under hypoxic stress,5-HT in the blood lymphocytes of O. reevesii responded and its content showed a trend of initial decrease,followed by an increase and then a decrease again. The injection of 5-HT resulted in higher survival rate of O. reevesii under hypoxic stress. After 9 hours of hypoxic stimulation, the survival rate of O. reevesii in the group injected with 5-HT was higher than that of the group injected with PBS and the hypoxic treatment groups. After O. reevesii under hypoxic stress for 9 h and was stimulated by 5-HT, it was found that the injection of exogenous 5-HT could reduce the oxidative stress levels,increase the activity and expression levels of antioxidant enzymes, and enhance the total antioxidant capacity. 【Conclusion】5-HT and 5-HTR in O. reevesii under hypoxic stress can respond and involve in physiological regulation.And the hypoxia tolerance and survival rate of O. reevesii can be increased by the injection of exogenous 5-HT.