The study compared electroporation efficiency of bipolar and unipolar nanosecond electric field oscillations NEFO. Bipolar NEFO was a damped sine wave with 140 ns first phase duration at 50 height the peak amplitude of phases 2-4 decreased to 35,12, and 7 of the first phase. This waveform was rectified to produce unipolar NEFO by cutting off phases 2 and 4. Membrane permeabilization was quantified in CHO and GH3 cells by uptake of a membrane integrity marker dye YO-PRO-1 YP and by the membrane conductance increase measured by patch clamp. For treatments with 1-20 unipolar NEFO, at 9.6-24 kVcm, 10 or 20 Hz, the rate and amount of YP uptake were consistently 2-3-fold higher than after bipolar NEFO treatments, despite delivering less energy. However, the threshold amplitude was about 7 kVcm for both NEFO waveforms. A single 14.4 kVcm unipolar NEFO caused a 1.5-2 times greater increase in membrane conductance p0.05 than bipolar NEFO, along with a longer and less frequent recovery. The lower efficiency of bipolar NEFO was preserved in Ca2-free conditions and thus cannot be explained by the reversal of electrophoretic flows of Ca2. Instead, the data indicate that the electric field polarity reversals reduced the pore yield.