During renal failure, polyamines and oxalate levels are elevated in the serum and the glomerular filtrate and are dumped by the kidney. Both of these compounds can be catabolized by oxidative reactions. We have, therefore, investigated the intracellular distribution of oxalate oxidase and of a polyamine oxidase in normal female rat kidney and liver. Polyamine oxidase was demonstrable, using spermidine as substrate in the cerous peroxyhydrate procedure of Briggs et al., in peroxisomes of kidney tubule cells and of hepatocytes. Oxalate oxidase could not be studied with this technique due to precipitation of cerium oxalate in the incubation medium. To demonstrate oxalate oxidase, and to confirm the polyamine oxidase localization, we incubated aldehyde-fixed tissue in a diaminobenzidine medium at pH 8, following the approach of Veenhuis et al., in which oxidases are demonstrated by virtue of their production of H2O2, which then serves as a substrate for endogenous catalase. Using oxalate or spermidine as substrate with this approach, we found reaction product in typical renal peroxisomes; we also found reaction product, with the polyamine substrate, in hepatocyte peroxisomes. To strengthen the conclusion that the oxidases themselves are present in peroxisomes, we used a light microscopic method, based on the tetrazolium procedures of Allen and Beard to demonstrate polyamine and oxalate oxidase activities in bodies with the distribution of renal peroxisomes.