To study the effects of gonadotropin on the expression of growth differentiation factor-9 (GDF-9) and -9B (GDF-9B) in mouse ovary. We chose follicles of mature mice cultured in vitro and mature BALB/c mice as our animal models. (1) In vivo experiment. Twenty mice were divided into two groups (groups A and B) randomly with ten mice in each group. Each mouse was injected with 10 IU pregnant mare's serum gonadotropin in group A and saline of the same volume was given to the other group. Twenty-four hours later we obtained ovarian tissue and immunohistochemistry and in situ hybridization were performed to detect the expression of GDF-9 and GDF-9B. (2) In vitro experiment. Fifty-eight separated follicles were divided into two groups (groups C and D) randomly and cultured in vitro. Thirty-two follicles in group C were cultured in medium with follicular stimulating hormone (FSH) for 72 hours while 26 were cultured without FSH. Immunohistochemistry was performed to detect the expression of GDF-9 in both groups. We ab