A sperm-induced intracellular Ca(2+) signal ([Ca(2+)](i)) underlies the initiation of embryo development in most species studied to date. The inositol 1,4,5 trisphosphate receptor type 1 (IP(3)R1) in mammals, or its homologue in other species, is thought to mediate the majority of this Ca(2+) release. IP(3)R1-mediated Ca(2+) release is regulated during oocyte maturation such that it reaches maximal effectiveness at the time of fertilization, which, in mammalian eggs, occurs at the metaphase stage of the second meiosis (MII). Consistent with this, the [Ca(2+)](i) oscillations associated with fertilization in these species occur most prominently during the MII stage. In this study, we have examined the molecular underpinnings of IP(3)R1 function in eggs. Using mouse and Xenopus eggs, we show that IP(3)R1 is phosphorylated during both maturation and the first cell cycle at a MPM2-detectable epitope(s), which is known to be a target of kinases controlling the cell cycle. In vitro phosphorylation studies reveal that MAPK/ERK2, one of the M-phase kinases, phosphorylates IP(3)R1 at at least one highly conserved site, and that its mutation abrogates IP(3)R1 phosphorylation in this domain. Our studies also found that activation of the MAPK/ERK pathway is required for the IP(3)R1 MPM2 reactivity observed in mouse eggs, and that eggs deprived of the MAPK/ERK pathway during maturation fail to mount normal [Ca(2+)](i) oscillations in response to agonists and show compromised IP(3)R1 function. These findings identify IP(3)R1 phosphorylation by M-phase kinases as a regulatory mechanism of IP(3)R1 function in eggs that serves to optimize [Ca(2+)](i) release at fertilization.