Since O2 is mainly consumed in muscle mitochondria during heavy physical work, one would expect to find a relationship between the volume density of mitochondria in skeletal muscles and maximal O2 uptake. We analyzed the volume density of mitochondria, Vv(mt,f) in four muscles of a series of African mammals ranging in body mass from 0.4 to 251 kg. Vv(mt,f) scaled as Mb-0.231, Mb-0.163, Mb-0.139 and Mb-0.055 in Mm. semitendinosus, longissimus dorsi, vastus medialis and diaphragm, respectively. The mass or volume of diaphragm was found to scale as Mb0.865, whereas for Mm. semitendinosus and vastus medialis, muscle volume (Vmu) scaled as Mb1.030 and Mb0.956 respectively. Scaling the absolute volume of mitochondria Vmt, in these muscles (Vmt = Vv (mt,f) x Vmu) against Mb gives regression lines whose slopes closely parallel that obtained for Vo2max against body mass. Therefore the ratio of volume of mitochondria in these muscles to Vo2max is body mass independent.