Skeletal muscle plasticity may expand aerobic capacity by either increasing capillary supply or altering fibre type composition. In addition, fibre size changes may influence intramuscular diffusion distances thereby affecting efficiency of oxygen delivery / metabolite removal. The aim of this study was to determine the effect of chronic voluntary wheel running exercise on oxygen transport capacity in a fast muscle, extensor digitorum longus (EDL) and its relation to activity characteristics. Voluntary wheel running imposes less stress on exercising animals than treadmill exercise, and is considered a better model of physiological adaptation. In addition, exercise volume may be adjusted according to energy intake. Male Wistar rats (n=9), body mass 360-397g were divided into three groups: running wheel (RW), RW with fructose supplement (RWF) and control (C). Animals had access to exercise wheels in individual cages and samples were taken after 7 weeks. EDL muscles were cryosectioned and immunohistochemically stained with Griffonia simplicifolia lectin I (Vector) to identify capillaries, and monoclonal anti-MHC antibodies to identify fibre type (BA-D5 for Type I and SC-71 for Type IIa; Developmental Studies Hybridoma Bank, University of Iowa). Images were captured at x20 to quantify capillary supply, mean fibre area, fibre type and capillary supply area (domains). RWF rats ran longer distances at higher running velocity compared to RW (9.90±0.63 vs. 3.79±0.20 km/day, P<0.05) with a similar number of activity bouts (P>0.05). Exercise alone (RW) stimulated a higher capillary to fibre ratio (C:F) relative to control (1.95±0.07 vs. 1.48±0.05, P<0.05), but increased exercise volume (RWF) produced no further increase (1.97±0.10, n.s.). In contrast, RWF produced an increased capillary density (CD) relative to both RW and C (956±198 vs. 784±7 and 718±99 mm-2, respectively, P<0.05) due to decreased mean fibre area. Regionally, CD was significantly higher in the glycolytic region of EDL from RW compared to C (750±84 vs. 609±38 mm-2, P<0.05), along with greater Type IIa numerical density (0.35±0.04 vs. 0.10±0.05, P<0.05). Oxygen transport capacity was assessed by frequency distribution of capillary domain area, and incorporation of fibre type distribution allows estimate of tissue PO2 distribution at rest and during mathematically modelled maximal oxygen consumption. It is concluded that exercise increases aerobic capacity through angiogenesis and muscle fibre type transformation, the extent of which is determined by the initial muscle fibre type composition.