Using a newly developed, extracellular vibrating electrode, we can now measure the electrical currents that traverse a single developing cell. We have studied the eggs of the common seaweed, Pelvetia, during their first 2 days of development and find that the endogenous electrical current through them includes a pulse component as well as a relatively steady component. Both of these enter the egg's growing tip and leave the rest of the embryo. The current pulses first appear a few hours after growth begins and have a characteristic shape that is independent of amplitude. They have a duration of about 100 sec, an average frequency of 1-5 per hr, and enter with peak surface intensities of 3-10 (and rarely up to 30) muA/cm(2). By the two-cell stage they account for about a fourth of the total transembryonic current. Since they may overlap to any degree and (as is documented elsewhere) are generally accompanied by peak membrane depolarizations of only 2-6 mV, their course does not seem to be voltage-controlled. Thus, they seem essentially different from action potentials. We also find that the rate at which the egg grows in length is roughly proportional to the size of the steady current traversing it.