In Pleurodeles waltl and Ambystoma mexicanum, which exhibit physiological polyspermy, the membrane potential in most eggs did not change in any consistent pattern during 45 min after fertilization; in some cases, a slow hyperpolarization began 5 to 15 min after insemination and continued for 10-15 min. These eggs then slowly depolarized, reaching a stable value of -10 to +10 mV, about 45 min after fertilization. Membranes of eggs activated by A23187 or by electrical stimulus showed a similar behavior. The diversity of responses does not correlate with the number of sperm fusing with the egg. Holding the membrane potential at a constant value between -40 and +40 mV during insemination did not prevent fertilization nor delay sperm-egg interactions. The fertilization or activation potential of Rana temporaria eggs consists of a rapid (1 sec) depolarization accompanied by a sudden decrease in membrane resistance. The activation potential can be triggered by A23187 and by calcium iontophoresis; its amplitude depends on the (Cl-)0 and to a lesser extent on the (Na+)0. Fertilization was prevented when the membrane potential was clamped above +15 mV. However, slowing the rise time (5 to 8 sec instead of 1 sec) and reducing the amplitude (10-20 mV instead of 40-60 mV) of the fertilization potential, both by injecting negative current, never induced polyspermy.