The electrical block to fertilization of sea urchin eggs can be overcome by very brief periods of inside-negative egg membrane potential. Lytechinus pictus eggs whose membrane potentials have been clamped at +15 mV cannot be fertilized. If the membrane potential is repolarized to inside-negative voltages for a brief interval, the egg can be successfully fertilized. By varying the duration and voltage of these brief periods of inside negativity, we have uncovered three general properties of the electrically sensitive step in fertilization. First, a membrane-potential step that becomes rate limiting at inside-positive voltages can be initiated within a few milliseconds of inside negativity (30-60 msec at -60 mV). Second, at the time that the electrically sensitive step is being completed, there are other potential-independent steps with probably slower time constants because the duration of negativity was more effective applied as paired pulses rather than a single long pulse. Third, the permissive state is more quickly established by inside negativity than the nonpermissive state is established by inside positivity because the interval between paired pulses could be a few times longer than the effective single pulse in duration. In these voltage-clamped eggs the intervals from the successful completion of the electrically sensitive step to the next identifiable signs of activation were on the order of several seconds and highly variable.