In vivo electro-transfection efficiency and manner of transferred gene expression were investigated by fluorescence microscopic image analysis. Green fluorescent protein (GFP) gene was used as the genetic marker. Electroporation was carried out on the liver of live rats by use of disk electrodes mounted in the tips of tweezers, which were directly pressed onto the surface of a liver lobe in situ. Electroporation with eight electric pulses of 50 ms in duration at 50 V gave a good efficiency of transfection as judged by the induced GFP expression. Bright fluorescence of GFP appeared as dots, which were scattered around the area damaged by electroporation. The transfection efficiency increased as the amount of injected DNA was increased. The results indicate that the amount of induced gene expression can be controlled. Estimation of the efficiency of electro-gene transfer using the fluorescence of GFP and digital analysis of microscopic images was useful to determine the optimum conditions for local gene therapy in tissues and organs.