The variability of structures supporting tissue oxygen transport (capillaries) and oxygen consumption (mitochondria) was analyzed in skeletal muscles of wildebeest and dik-dik. Regional differences in mitochondria and capillary densities within individual muscles were found for M. semitendinosus (twofold) but not for M. longissimus dorsi and diaphragm. Comparing 20 different muscles from both animals, the volume density of mitochondria in the muscle fibers [Vv(mt,f)] was significantly higher in diaphragm (10-12%) and varied considerably (1-6%) in the other muscles. The relation between Vv(mt,f) and the number of capillaries per cross-sectional fiber area NA(c,f) showed great variability. In glycolytic fibers Vv(mt,f) was typically low (1%) whereas in oxidative fibers it ranged from 5-15%. No systematic trend was found for the packing of cristae in subsarcolemmal and interfibrillar mitochondria from both types of fibers in large and small animals.