A surge of luteinizing hormone (LH) from the pituitary gland triggers ovulation, oocyte maturation, and luteinization for successful reproduction in mammals. Because the signaling molecules RAS and ERK1/2 (extracellular signal–regulated kinases 1 and 2) are activated by an LH surge in granulosa cells of preovulatory follicles, we disrupted Erk1/2 in mouse granulosa cells and provide in vivo evidence that these kinases are necessary for LH-induced oocyte resumption of meiosis, ovulation, and luteinization. In addition, biochemical analyses and selected disruption of the Cebpb gene in granulosa cells demonstrate that C/EBPβ (CCAAT/Enhancer-binding protein–β) is a critical downstream mediator of ERK1/2 activation. Thus, ERK1/2 and C/EBPβ constitute an in vivo LH-regulated signaling pathway that controls ovulation- and luteinization-related events.