Electroporation, the permeabilization of the cell membrane with electrical pulses, is being used for in vivo gene therapy, drug therapy and minimally invasive tissue ablation. Applying electrical pulses across cells can have a variety of outcomes; from no effect to reversible electroporation to irreversible electroporation. For reliable in vivo use of electroporation it is important to have real time feedback on the outcome of the application of the electrical pulse. Recently, it has been proposed that measuring the electrical properties of electroporated tissues in temporal relation to the applied pulses could provide this feedback. To generate fundamental data on the in vivo electrical properties of electroporated tissues we have developed a fast spectroscopic impedance analyzer that measures electrical properties of tissues in vivo in conjunction with commercial electroporation pulse generators. Here we describe the apparatus and illustrate its use with an experiment on reversible electroporation in a rat liver.