In this study, the changes in interleukin-1β (IL-1β) mRNA expression and its localization in the ovary were studied in pregnant mares' serum gonadotropin (PMSG)-human CG (hCG) injected immature rats. Moreover, to study the in vivo role of IL-1 during ovulation, the IL-1 receptor antagonist (IL-1ra) was injected into the bilateral bursae (100μg/15μl/bursa) immediately or at the 3rd, 5th, 10th h after hCG injection. IL-1β mRNA expression in the ovary had significantly (P<0.05) increased at the 6th h after hCG injection, and its level depended on the time interval between PMSG and hCG injections; the maximal IL-1β mRNA expression was observed when hCG was injected 48h after the PMSG injection. The signals of IL-1β mRNA localized in the thecal layer, particularly in the large preovulatory follicles. Intrabursal IL-1ra injection at the 3rd h after hCG injection showed a trend toward lowering the number of ova shed, and the injection at the 5th h significantly (P<0.01) decreased the number of ova shed compared with the saline injected control group. At the 24th h after hCG injection, there were a few large preovulatory follicles with distinguishable theca and granulosa layers in the IL-1ra treated ovary, although these two layers were indistinguishable in the control ovary. These results indicated that IL-β mRNA expression was definitely induced by hCG stimulation, and, moreover, this expression was dependent on the stage of follicular maturation. IL-1 may be important for the ovulation in the superovulatory cycle by PMSG in rats.