It has been demonstrated that a massive Ca2 release in response to InsP3 results in elevation of the vitelline layer in starfish oocytes. As expected, the control eggs responded to threerepeated InsP3 uncagings by showing three sharp rises of intracellular Ca2. Immediately after the first Ca2 transient, the cell responded with the elevation of vitelline layer as a result of cortical granule exocytosis.In the presence of U73122,which increased cortical F-actin the Ca2 transient in response to the first InsP uncaging was slightly lower than in the control. However, these cells responded to the following InsP3 uncaging by showing the same magnitude of Ca2 signals as in the control(Fig. 7c). The main difference between the control and the U73122-incubated cells was in the fact that the elevation of the vitelline envelope was totally blocked in the U73122-treated cells. Hence, despite the massive release of intracellular Ca, U73122 could still block the exocytosis of cortical granules. Supporting the idea that the blockade of cortical granule exocytosis by U73122 is linked to the enhancement of the cortical actin layer. another pharmacological agent inducing cortical actin polymerization, AS, also produced basically the same results in independent experiments.