Experiments were performed to identify the ionic basis of membrane potential changes in the Rana cameranoi egg, from prior to fertilization through the first cleavage. The membrane potential was monitored continuously during this period. Ten per cent Ringer was used as the recording solution in the control group. The effects of Na+ or Ca2+ conductances were observed by altering external concentrations of these ions. K+ and Cl- conductances were tested by adding channel blockers of these ions (TEA and SITS, respectively) to the extracellular medium.