Leydig cell tumors of the rat testis induced by a single subcutaneous injection of cadmium chloride were studied by electron microscope and histochemical methods. The tumor cells contained abundant agranular endoplasmic reticulum appearing as a network of interconnecting tubules of uniform diameter. The mitochondria were numerous, varied in size, and contained compact vesicular inner structure. In some tumor cells the mitochondrial cristae were not elaborate. The cytoplasmic lipid appeared as numerous vacuoles and was more prominent in larger tumors. Many microbodies (peroxisomes) were also observed in tumor cells. Some possessed tubular or cylindrical inclusions identical to those in peroxisomes of kidney and spontaneous Leydig cell tumors of the rat testis. Several microbodies also revealed the presence of an amorphous, highly electron dense center. 3β-Hydroxy steroid dehydrogenase activity was localized in these tumors histochemically. These observations indicate that cadmium-induced Leydig cell tumors in the rat testis are capable of steroidogenesis.