TRANSIENT changes in intracellular calcium ([Ca2+](i)) have been shown to punctuate the cell cycle in various types of cells in culture(1-5) and in early embryos(6-12). The [Ca2+](i) transients are correlated with cell-cycle events : pronuclear migration, nuclear envelope breakdown, the metaphase-anaphase transition of mitosis, and cytokinesis. Mitotic events fan be induced by injecting calcium and prevented by injecting calcium chelators into the sea urchin embryo(10,13). Cell-cycle calcium transients differ from the transients linked to membrane signal transduction pathways : they are generated by an endogenous mechanism, not by plasma membrane receptor complexes, and their trigger is unknown. We report here that the phosphoinositide messenger system oscillates during the early embryonic cell cycle in the sea urchin, leading to cyclic increases in inositol trisphosphate that trigger cell-cycle [Ca2+](i) transients and mitosis by calcium release from intracellular stores.