The musculature of small lung veins, of the thoracic portion of the inferior vena cava, and of other thoracic veins of the mouse have been studied in the electron microscope. Tissues were fixed in 1 per cent osmium tetroxide buffered with veronal, to which either sodium chloride or sucrose had been added. Methacrylate or araldite served as embedding matrices. Phosphotungstic acid or uranyl acetate was used to stain some of the preparations. Thin sections were examined in a Siemens and Halske Elmiskop Ib electron microscope. The entire musculature of the veins examined was of the striated type. It represents a variety of cardiac muscle, characterized by centrally located nuclei, typical mitochondria, and narrow I bands. Many I bands cannot be recognized at all. H and M bands are likewise indistinct. There is a double array of primary and secondary myofilaments. Mitochondria are large and numerous and contain many cristae. The endoplasmic reticulum consists of longitudinal tubules which run through the whole sarcomeres and bypass Z bands, and of transverse tubules which accompany Z bands. Some "triads," located at Z levels, consist of flattened vacuoles flanked by such transverse tubules. Small vesicles located at Z bands, close to the nucleus, and beneath the plasma membrane may represent still other portions of the reticulum.