THE fertilization of the sea urchin egg starts a sequence of changes in the cell membrane which can be measured as changes of membrane potential and membrane resistance1-3. The immediate consequence of contact of ovum and spermatozoon is an action potential, a depolarization and repolarization including a transient reversal of polarity. The overall sequence1,2 converts the anion-permeable membrane of the unfertilized egg (with a potential of about -10 mV, inside negative) to a rather typical K+ permeable membrane (about -60 mV, inside negative). The same results are obtained with parthenogenetic activation of the egg.