The angio-architecture of normal, hypertrophic and denervated rat muscles was studied by micro-angiography and by scanning electron microscopy of vascular casts made of methylmethacrylate. In hypertrophy, there was a slightly coarse-meshed irregular capillary network with some loss of normal parallelism, an increased calibre and a slight spiralization of all vessels. Probably all these changes, except the slight spiralization, increase the circulation. In denervation atrophy, the vascular pattern was much more irregular, with much more marked spiralization and loss of parallelism and narrowed capillaries. The changes probably contribute to the impairment of the circulation.