This study showed that the vascular pattern of the spontaneous C3H mouse mammary carcinoma develops from a capillary network into an afferent system lacking arterioles and consisting only of capillary-like vessels and an efferent system characterized by large sinuses. Lack of correlation between the growth of stroma and parenchyma leads to a circuitous and uneven supply of blood and to a high degree of occlusion of the efferent system with consequent reduction in the rate of flow of blood. The parenchyma consists of tubules formed of single or multiple layers of cells between which capillaries do not penetrate. The diffusion pathway of oxygen and nutrients to the inner cells of the multi-layered tubules is considerably longer than that to their outer cells or to the cells of the single-layered tubules. Consequently it is in the former parts that anoxia and severe hypoxia are likely to prevail. The pattern of necrosis agrees with this supposition. It is predicted that radiation hyperthermia will act differentially and in opposite senses on these two tumour components, hyperthermia being more effective on the former, radiation on the latter.