Peroxidase has several roles in each organ. In neural tissue, it is considered from pathological examination to be related to the aging process. To study biochemically the presence of peroxidase, diaminobenzidine was used as the hydrogen donor and the enzyme was partially purified from rat brain and characterized. Diaminobenzidine peroxidase was purified 670-fold from the original homogenate. The presence of p-phenylenediamine and guaiacol peroxidase was not detected in either the crude homogenate or the partially purified fraction. The possible presence of isozyme(s) in peroxidase was implied and this may be related to the physiologic differences in organs.