The electrochemical behavior of porcine superoxide dismutase (PESOD) on mercury electrodes was investigated by cyclic voltammetry and direct current polarography in the absence of mediators for the first time. It is interesting and abnormal that PESOD displays two pairs of redox adsorbed peaks in the CV diagram, which are located in the potential regions of the disulfide bonds of many proteins reported by some authors, but we attributed them to the redox peaks of copper(II) and zinc(II) by reconstitution of PESOD. The apparent numbers of electrons in the reduction process were obtained by polarography and the electrode process was discussed.