The effective membrane capacity (Ceff) of the Xenopus egg has been measured integrating the membrane current transients in response to small voltage-clamp pulses. Before activation Ceff has a value of 1.34 +/- SE. 0.13 microF/cm2 (apparent surface area, 13 eggs from 3 females) and is essentially constant over the voltage range between - 30 and + 60 mV. During artificial activation of the eggs by pricking or by addition of Ca2+ ionophore A23187, Ceff increases by about 60% in 2-3 min and then slowly decreases returning to near the initial value in 15-20 min. Electron microscopic observations of the egg surface at different times reveal that the capacity time course parallels the changes in plasma membrane area due to cortical granule exocytosis and to a later reduction of microvillar extension. Simultaneous measurements of capacity and conductance show that the capacity changes are slower and delayed in comparison with the transient development of the chloride conductance responsible for the activation potential. In CO2-treated eggs the cortical granule exocytosis is prevented and, correspondingly, the transient capacity increase is strongly reduced or absent, but the development of the chloride conductance remains normal. This technique gives a method to electrophysiologically monitor the cortical granule exocytosis; moreover our results show that the exocytotic process can be blocked without affecting the membrane conductance changes.