Hamster sperm extract (SE) possessing Ca 2+ oscillation-inducing activity was microinjected into the peripheral or central region of mouse eggs, and the first increase in intracellular Ca 2+ concentration ([Ca 2+ ] i ), together with the spread of fluorescence-labeled SE in the ooplasm, was investigated by imaging with confocal microscopy. Injection into the periphery always induced a Ca 2+ wave that started from the injection site after a delay of 5 to 30 s depending on the concentration of SE. The diluted SE caused a wave of two-step [Ca 2+ ] i rises, which was always observed at fertilization. Injection into the center could induce a radial Ca 2+ wave with relatively high dose of SE, but lower dose of SE caused a [Ca 2+ ] i rise after a longer delay which was initiated synchronously over the ooplasm or was preceded in a peripheral area. Injection of diluted SE remarkably prolonged the delay time and reduced the rate of [Ca 2+ ] i rise. The critical concentration of SE needed to induce [Ca 2+ ] i rise was significantly lower in the periphery. These results indicate that the sensitivity to SE is higher in the cortex. SE-induced [Ca 2+ ] i rises were blocked by an antibody against the type 1 inositol 1,4,5-trisphosphate receptor (InsP 3 R). The cortex was substantially more sensitive to injected InsP 3 induction of Ca 2+ release than the center. It is suggested that the cortex of mouse eggs may involve a functionally specialized organization of InsP 3 Rs and Ca 2+ pools in which a cytosolic sperm factor(s) could act upon sperm–egg fusion to cause Ca 2+ release, leading to the Ca 2+ wave at fertilization.