Fertilization of a mouse oocyte triggers oscillations of the concentration of free calcium ions in ooplasm ([Ca2+] i ). Calcium oscillations are generated in a process involving inositol 1,4,5-triphosphate (IP 3 ) produced by phospholipase C zeta (PLC zeta), a sperm specific isoform of phospholipase C. Calcium transients last for several hours until the time of pronuclei formation. The ability to generate and sustain long-lasting oscillations of [Ca2+] i is developed in oocytes during meiotic maturation, i.e.