Although activation of a sea urchin egg by sperm leads to three phases of membrane conductance increase in the egg, the mechanism by which the sperm causes these conductance changes is not known. We used the loose patch clamp technique to localize the conductance changes in voltage clamped eggs. A patch of the egg's membrane was isolated from the bath by pressing the loose patch clamp pipette against the egg surface. Sperm added to the bath attached to the surface of the egg in a region other than at the isolated membrane patch. During phase 1 of the activation current, no changes of the membrane conductance were detected. At the time of, and subsequent to the onset of phase 2, large currents recorded between the interior of the patch pipette and the bath were attributed to changes of the seal resistance between the surface of the egg and the pipette. A local change of membrane conductance was observed during phase 2 despite the changes of seal resistance. During phase 2, the large amplitude and short duration of the local membrane conductance increase relative to the membrane, conductance increase for the whole egg during phase 2 indicated that the conductance increase occurred over the entire surface of the egg, but not simultaneously. The time when the peak conductance for the membrane patch occurred, relative to the time of onset for phase 2 in the whole egg, depended on the distance, measured in a straight line, between the site of sperm attachment and the tip of the pipette. These data indicate that the localized conductance increase progressed over the surface of the egg from the site of sperm attachment to the opposite pole of the egg. It is proposed that the local conductance increase, the cortical reaction, and the change of seal resistance are all evoked by a common cytoplasmic message that progresses throughout the cytoplasm of the egg from the site of sperm attachment to the opposite pole of the egg.