Fertilization of the immature, prophase I-arrested mouse oocyte produces multiple Ca²⁺ transients similar to those of the mature, metaphase II egg; however, the first Ca²⁺ transient is much lower in amplitude and shorter in duration. In contrast to prophase I-arrested oocytes, maturing oocytes fertilized after germinal vesicle breakdown have first Ca²⁺ transients similar to those of mature fertilized eggs. Immature, prophase-arrested oocytes release less Ca²⁺ in response to injection of inositol 1,4,5-trisphosphate (IP₃) than eggs. At high concentrations, the sulfhydryl reagent, thimerosal (200 μM), causes Ca²⁺ oscillations in eggs and produces similar oscillations in oocytes. A lower concentration of thimerosal (25 μM) does not cause Ca²⁺ oscillations, but does sensitize IP₃-induced Ca²⁺ release in both eggs and oocytes, since IP₃-induced Ca²⁺ release is enhanced in the presence of 25 μM thimerosal. Incubation of oocytes in 25 μM thimerosal before injection of 2.2 μM IP₃ causes oocytes to release as much Ca²⁺ as is released in eggs injected with 2.2 μM IP₃. These results indicate that immature mouse oocytes possess intracellular stores of releasable Ca²⁺ similar in size to Ca²⁺ stores in eggs; however, these stores are less sensitive to IP₃. Development of the IP₃-induced Ca²⁺ release mechanism may be an important component of maturation; at fertilization of the egg, Ca²⁺ must be elevated to levels sufficient to activate further development and establish a block to polyspermy. Mouse oocytes appear to develop an increased sensitivity to IP₃ during the course of oocyte maturation.