The blood supply of spontaneously occurring pulmonary metastases in dogs was studied using magnification angiography and postmortem barium sulfategelatin injection. Serial observation was simulated by analysis of lesions with progressively greater parenchymal and vascular changes. Irregularly dilated pulmonary vessels were seen within many small metastases and around the margins of larger ones. Tumor tissue infiltrating intact alveoli may have received its circulation from the pulmonary capillaries. All new blood vessels were systemic in origin, associated with bronchial arteries or transpleural collaterals. Drugs infused into the bronchial arteries therefore would not reach the fastgrowing peripheral portions of metastases; pulmonary arterial infusions would not reach viable neovascularized regions. Angiographic interpretation should take account of the non-specific hypervascularity that surrounds neoplasms, resulting particularly from capillary and venous dilatation. The most conspicuous “tumor vessels” are dilated host vessels around the tumor, and the term “neovascularity” should be reserved for demonstrably new vessels.