The transmembrane potential (A<f>) induced by external electric fields is important both in biotech applications and in new medical therapies. We analyzed the effects of AC field frequency and cell orientation for cells of a general ellipsoidal shape. Simplified equations were derived for the membrane surface points where the maximum Atf> is induced. The theoretical results were confirmed in experiments with three-axial chicken red blood cells {a\b\c=6.66 (jm:4.17 (jm:1.43 pm). Propidium iodide (PI) staining and cell lysis were detected after an AC electropermeabilization (EP) pulse. The critical field strength for both effects increased when the shorter axis of a cell was parallel to the field, as well as at higher field frequency and for shorter pulse durations. Nevertheless, data analysis based on our theoretical description revealed that the Atf> required is lower for the shorter axis, i.e. for smaller membrane curvatures. The critical Atf> was independent of the field frequency for a given axis, i.e. the field strength had to be increased with frequency to compensate for the membrane dispersion effect. Comparison of the critical field strengths of PI staining in a linear field aligned along semi-axis a (142 kv rrf and a field rotating in the a-b plane (115 kv rrf revealed the higher EP efficiency of rotating fields.