An early single dose of progesterone agonist attenuates endogenous progesterone surge and reduces ovulation rate in immature rat model of induced ovulation

► Gonadotropin-treated rat follicles expressed StAR and 3β-HSD mainly in thecal layer. ► An early single dose of exogenous progesterone agonist could inhibit ovulation rate. ► Progesterone agonist attenuated endogenous progesterone surge and StAR induction. ► The treatment reduced granulosa COX-2 induction and ovarian PGE2 content. ► Properly regulated progesterone is an early critical mediator of ovulatory cascade.