Mature hamster eggs exhibit repetitive increases in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) at fertilization, caused by inositol 1,4,5-trisphosphate (InsP₃)-induced Ca²⁺ release (IICR) from stores. Oscillating Ca²⁺ rises also occurred in inseminated immature oocytes at the germinal vesicle (GV) stage but the peak [Ca²⁺]_i of each response was about half of that in mature eggs. Like the responses to sperm in the mature egg, those in the immature oocyte were also blocked by injection of a monoclonal antibody against the InsP₃ receptor (18A10). Compared with mature eggs, immature oocytes were more sensitive to externally applied serotonin which caused repetitive IICR, but each response was smaller. The development of the IICR mechanism during oocyte maturation was investigated using iontophoretic injection of InsP₃ and Ca²⁺ imaging with fura 2. In immature oocytes the rise in [Ca²⁺]_i with an InsP₂ pulse of 1.2 nA for 1 sec was 35% of that in mature eggs. The response increased with increasing doses of InsP₃. The nearly maximum response, obtained with a 20-nA 2-sec pulse, was 80% of that in mature eggs, suggesting that immature oocytes are less sensitive to InsP₃ but that most of Ca²⁺-releasable stores are already present in the immature oocyte. IICR developed in two phases during in vivo maturation. The sensitivity to InsP₃ increased gradually between the GV stage and prometaphase of the first meiosis. Then a nonlinear dose-response relation developed during the second meiosis to metaphase II, resulting in regenerative, propagating Ca²⁺ release induced by a threshold pulse of 1 nA for 1 sec in mature eggs. Similar changes occurred during in vitro maturation of oocytes isolated from follicles at GV stage and cultured for up to 16 hr. The development of IICR is thought to be a prerequisite factor for the acquisition of the ability of an egg to undergo normal fertilization.