Abstract: This experiment was to clarify the change of sex steroid hormone in serum of loach(Misgurnus anguillicaudatus) infected LHRHa,and investigate actions of progesterone(P),testosterone(T) and estradiol(E2) to gonadal development and regulatory mechanism.It contented two groups,control group and test group.The fish of control group were only injected physiological saline,and the fish of test group were injected chorionic gonadotropin analogue D-Ser(tBu)6,Pro9-NEt(LHRHa)(female fish 0.2ug/g and male fish 0.1ug/g).Blood samples were collected from caudal vein at 0,7,24,48,72 and 96h after implantation respectively,and T,E2 and P contents in serum were measured.These results showed that T and E2 in serum of female fish infected LHRHa were significantly higher than those of control group(P0.05).24h after infection LHRHa,P,T and E2 contents of female fish were 0.710±0.082ng/mL,9.00±0.57ng/mL and 696.4±26.2pg/mL respectively;P and T of male fish were 0.527±0.121ng/mL and 9.62±0.62ng/mL respectively.Change discipline of female fish in test group was as following rule,basis level(0—7h)-to rise gradually to the peak value(7—24h) to go down to basis level(24—48h) to maintain basis level(48—96h).The change discipline of T and E2 in female fish and P and T in male fish were similar,to rise gradually to the peak value(0—24h) to go down gradually to basis level(24—48h) to maintain basis level(48—96h).After 24h,the T and E2 contents of female fish in control group been significantly hoisted to 2.20±0.18ng/mL and 269.1±36.6pg /mL,but the P and T contents of male fish in control group changed unobviously(P0.05).The experimental results indicated that LHRHa could stimulate sex steroid hormone secretion of loach to female loach,especially T,and raised significantly the gonadosomatic index(GSI),but it had poor effect to induce ovulation.The reason may be that it had especial regulatory mechanism of steroid hormone.E2 and T in loach could promote sex gland maturity.P may play important role in final maturity of oocyte and ovulation in loach,and E2 and T had no obvious action in this way.