Abstract: AIM: To study the inhibitory effect of alarelin(D-Ala⁶, Pro⁹NHEt) microspheres(LHRH-Ams) on the growth of endometrial explants in rat model of endometriosis. METHODS: The model of endometriosis in rats was made by surgically autotransplanting endometrium to the peritoneum. After 4 weeks, the rats were laparotomized and randomly assigned to 4 groups: control group; alarelin lyophilized power(LHRH-Alp) group (daily subcutaneous injection); LHRH-Ams group(doses of 20,100,200 μg.kg^-1.d^-1 to group 1,2,3) were administered and castration group. In addition, blood samples were collected at different times to measure serum E₂ level. RESULTS: The results showed that chronic administration of LHRH-Ams for 4 weeks produced obvious inhibitory effect on the growth of endometrial explants in rats in dose-dependent manner. The inhibitory effect of LHRH-Ams might be equal to that of ovariotomy at a dose of 100 μg.kg^-1.d^-1. At the same time, an obvious depression in estradiol serum level occurred, the levels was equivalent to those in diestrous rats. This inhibitory effect was associated with the reduction of serum estradiol concentration. CONCLUSION: the effect of LHRH-Ams on experimental endometriosis is similar to those of LHRH-Alp. LHRH-Ams decrease serum E2 level and produce obvious inhibitory effect on endometrial explants in rats, which might even be equal to that of ovariotomy.