In the present review, we attempt to summarize our recent research related to comparative effects of gonadotropin-releasing hormone (GnRH) agonist on the proliferation, apoptosis and differentiated function of cultured porcine granulosa cells from varying follicular stages. The inhibitory effect of GnRH agonist on Proliferating cell nuclear antigen positive rate of cultured cells was prominent in granulosa cells from small and medium but not from large follicles. By contrast, the inhibitory effect of GnRH agonist on 17beta-estradiol and progesterone secretion by cultured cells was prominent in granulosa cells from large but not from small and medium follicles. The stimulatory effect of GnRH agonist on apoptosis positive rate of cultured cells was, however, uniform regardless of the stages of follicular growth. These results demonstrate that GnRH agonist exerts diverse actions on granulosa cells over the course of follicular growth. One downregulates granulosa cell proliferation in immature follicles as well as steroidogenesis in mature follicles, and the other upregulates apoptosis of granulosa cells regardless of the stages of follicular growth.