The activating hamster egg shows a series of periodic increases in intracellular free calcium concentration ([Ca2+]i) (Igusa and Miyazaki, 1986). The spatial distribution of the Ca2+ transients can be analyzed in single, aequorin-injected eggs by the photon-counting imaging method with a supersensitive TV camera system (Miyazaki et al., 1986). A propagating increase in [Ca2+]i is observed in the first response, starting from the sperm attachment site and spreading over the entire egg within 4-7 s. The Ca wave is repeated in the second and sometimes the third response, starting from the same focus, but spreading more rapidly (∼2 s). In subsequent responses, [Ca2+]i increases synchronously in the whole egg within 1 s. A Ca2+ transient is induced in an all-or-none fashion by injection of either inositol 1,4,5-trisphosphate (IP3) or Ca2+ ions into unfertilized eggs. The Ca2+ transient is due to release of Ca2+ from intracellular stores, and it is followed by a refractory period of 60-120 s. Injection of guanosine-5'-0-(3-thiotriphosphate) (GTPγS) causes a series of Ca2+ transients with a pattern similar to that upon insemination, but with remarkable attenuation of the [Ca2+]i rise after 2-3 responses. Signal transduction involving activation of a GTP-binding protein and IP3- and/or Ca2+ -induced Ca2+ release is considered to be responsible for the repeated Ca2+ transients.