The universal signal for egg activation at fertilization is a rise in cytoplasmic Ca 2+ with defined spatial and temporal kinetics. Mammalian and amphibian eggs acquire the ability to produce such Ca 2+ signals during a maturation period that precedes fertilization and encompasses resumption of meiosis and progression to metaphase II. In Xenopus, immature oocytes produce fast, saltatory Ca 2+ waves that can be oscillatory in nature in response to IP 3. In contrast, mature eggs produce a single continuous, sweeping Ca 2+ wave in response to IP 3 or sperm fusion. The mechanisms mediating the differentiation of Ca 2+ signaling during oocyte maturation are not well understood. Here, I characterized elementary Ca 2+ release events (Ca 2+ puffs) in oocytes and eggs and show that the sensitivity of IP 3-dependent Ca 2+ release is greatly enhanced during oocyte maturation. Furthermore, Ca 2+ puffs in eggs have a larger spatial fingerprint, yet are short lived compared to oocyte puffs. Most interestingly, Ca 2+ puffs cluster during oocyte maturation resulting in a continuum of Ca 2+ release sites over space in eggs. These changes in the spatial distribution of elementary Ca 2+ release events during oocyte maturation explain the continuous nature and slower speed of the fertilization Ca 2+ wave.