The relationship between capillary density and mitochondrial volume density in skeletal muscle tissue is investigated on 25 African mammals ranging in body mass from 0.4 to 251 kg. As a general trend higher capillary densities, NA (c,f) are found in muscles with higher volume densities of mitochondria, Vv(mt,f). The individual data however show considerable scatter. Part of this scatter can be explained by the fact that NA (c,f) alone is not a sufficient parameter to account for functional properties of the vascular supply. The regional variability of the capillary network from arterial to venous end also contributes to the data spread. A number of physiological variables of the vascular compartment may also influence the relationship of capillary counts to the mitochondrial densities of muscle fibers. This relationship therefore cannot be expected to be a simple one.