Under the influence of strong electric fields the permeability of tumor cell membranes to poor permeating drugs increases and as a result the tumor growth is inhibited. This new tumor treatment method is named electrochemotherapy. We investigated the electrochemotherapy influence of bleomycin upon mice Lewis Lung Carcinoma by using optical histological and electron microscopic tumor analysis. It was shown that due to electrochemotherapy tumor necrosis area is significantly increased, intercellular gaps enlarge, and a big amount of cells is destroyed. Cell and nucleus membranes break as well as hemorrhage are very often. The observable histological tumor change was noted when electrical pulses of 1300 V/cm and 0.1 ms duration were applied. Pulses of 1700 V/cm and 0.1 ms duration induce total tumor destruction.