Intracellular Ca2+ oscillations in fertilized mammalian eggs, the key signal that stimulates egg activation and early embryonic development, are regulated by inositol 1,4,5-trisphosphate (IP3) signaling pathway. We investigated temporal changes in intracellular IP3 concentration ([IP3](i)) in mouse eggs, using a fluorescent probe based on fluorescence resonance energy transfer between two green fluorescent protein variants, during Ca2+ oscillations induced by fertilization or expression of phospholipase C zeta (PLC zeta), an egg-activating sperm factor candidate. Fluorescence measurements suggested the elevation of PPA in fertilized eggs, and the enhancement of PLC-mediated IP3 production by cytoplasmic Ca2+ was observed during Ca2+ oscillations or in response to CaCl2 microinjection. The results supported the view that PLC zeta is the sperm factor to stimulate IP3 pathway, and suggested that high Ca2+ sensitivity of PLC zeta activity and positive feedback from released Ca2+ are important for triggering and maintaining Ca2+ oscillations. (c) 2006 Elsevier Inc. All rights reserved.