Application of an electron-microscopic histochemical staining method to rat uterus demonstrated endogenous peroxidase activity in erythrocytes, blood and tissue eosinophils and in the uterine epithelium. The reaction in the epithelium was localized in the endoplasmic reticulum and nuclear envelope. Under certain conditions a positive reaction, possibly attributable to cytochrome oxidase was also present in the intracristal space and between the limiting membranes of the mitochondria.