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SUMMARY1. Fertilization or artificial activation of the frog egg (Rana pipiens) elicits a positive-going shift in membrane potential which results from an opening of Cl-and K+ channels in the egg membrane. We examined the spatial localization of the currents produced by the opening of these channels, using large patch electrodes (tip diameters 3-10 ,um). We also used small patch electrodes (tip diameters about 1 /Im) to study the single K+ channel currents.2. After activation, with the patch electrode at any position on the egg surface, we observed a transient current, with a main peak lasting several seconds. This activation current occurred after a variable delay of 0-3 min following the rise of the activation potential. With 10 % Ringer solution in the bath and pipette, the current was usually outward, although it sometimes had an inward component.3. With one patch electrode on the animal surface of the egg and another patch electrode on the vegetal surface, we observed that the activation current propagated over the egg surface. In experiments where the egg was activated by applying a hyperpolarizing pulse, the response in the animal half preceded that in the vegetal half by an average of about 1 min. The amplitude of the peak outward current was similar for animal and vegetal recordings (1-2 mA/cm2).4. Tetraethylammonium (11 mM) in the patch pipette blocked most ofthe outward component of the activation current and revealed an underlying inward component.5. The inward component of the activation current was carried by Cl-, since it could be reversed by raising the Cl-concentration in the pipette.6. The Cl-component of the activation current propagated over the egg surface, with timing similar to that of the total current. 7. The average amplitude of the peak Cl-current was six or more times larger at the animal than the vegetal surface.8. Fertilization caused a current to propagate from the animal to the vegetal surface, like the current observed during activation.9. With a small patch electrode, single channel currents of the K+ component of the activation current could be seen. The probability that the channels were open increased at more positive potentials. The single channel conductance was estimated 8-2 228 L. A. JAFFE, R. T. KADO AND L. MUNCY to be 25 pS, and the reversal potential to be -150 mV. Single Cl-channel currents have not yet been seen.10. Activation or fertilization of the frog egg resulted in a wave-like opening of Cl-and K+ channels, which spread from the animal to the vegetal half of the egg. The Cl-conductance was larger on the animal half.