ABSTRACT Iontophoresis of inositol 1, 4, 5-trisphosphate into frog (Xenopus laevis) eggs activated early developmental events such as membrane depolarization, cortical contraction, cortical granule exocytosis, and abortive cleavage furrow formation (pseudocleavage). Inositol 1,4-bisphosphate also triggered these events, but only at doses -100-fold higher, whereas no level of fructose-I, 6- bisphosphate tested activated eggs. Using Ca2+-selective microelectrodes, we observed that activat- ing doses of inositol 1, 4, 5-trisphosphate triggered a Ca 2+ release from intracellular stores that was indistinguishable from that previously observed at fertilization (Busa, W. B., and R. Nuccitelli, 1985r J. Cell Biol., 100:1325-1329), whereas subthreshold doses triggered only a localized Ca 2÷ release at the site of injection. The subthreshold,IP3 response,could,be distinguished from the major Ca 2÷ release at activation with respect to their dose-response characteristics, relative timing, sensitivity to external Ca 2+ levels, additivity, and behavior in the activated egg, suggesting that the Xenopus egg may,possess two functionally distinct Ca 2+ pools mobilized,by different effectors. In light of these differences, we suggest a model for intracellular Ca 2+ mobilization by sperm-egg interaction. A transient increase in intracellular free Ca 2÷ concentration