In order to study membrane charging in mammalian cells during exposure to pulsed electric fields of 60 ns duration we measured transmembrane voltage changes during and after exposure in real-time, i.e. with a resolution that is short compared to the duration of the administered electrical pulse. The applied electric field was varied between 5 kV/cm and 90 kV/cm. Under all conditions the voltage at the hyperpolarized pole of the cell is changing by more than 1 V during the first 5 ns of the exposure. A further hyperpolarization of the membrane of up to 1.6 V depends on the strength of the applied field. A change of at least 1.4 V at the anode will cause pores to open and allow ion exchange. Immediately after this maximum is reached, potential differences start to readjust. In principle, voltages at the depolarized pole follow the same pattern. However, the change is, in general lower by 1 V, limiting the depolarization to a maximum of 0.6 V.