Zona-free rat oocytes inseminated with capacitated sperm, under conditions that allow polyspermic fertilization, exhibited a rapid, transient elevation of cellular calcium (from 147 +/- 10 to 607 +/- 55 nM, n = 19, measured by Fura 2 fluorescence ratio imaging) immediately after sperm attachment. This peak was followed by a series of dramatic calcium transients of high amplitude (maximal 847 +/- 32 nM) and frequency (range 2.1 +/- 0.07 - 3.9 +/- 0.07 min), which continued for several hours. A similar pattern was seen also in zona-free oocytes fertilized with low sperm density (i.e. producing mainly monospermic attachment) and in zona-enclosed oocytes fertilized in vitro. Moreover, single or repetitive calcium transients were observed in rat oocytes fertilized in vivo. These findings indicate that in normal fertilization in vivo, sperm-oocyte interaction initiates a prolonged train of cyclical calcium changes in the oocyte. This activity may be necessary for the early events in the fertilization process.