The anatomic size of the capillary-to-fiber (C/F) interface plays an important role in O(2) flux from blood to tissue by determining the surface area available for diffusion and is maintained in relative proportion to fiber mitochondrial volume across a wide range of muscle aerobic capacity. In the present study, we examined an estimate of the anatomic size of the C/F interface [the quotient of the individual C/F ratio and fiber perimeter, C/F perimeter exchange (CFPE) index] and fiber oxidative capacity in different skeletal muscles, or muscle regions, to test the hypothesis that capillarization would be maintained in relative excess of reduced fiber oxidative capacity in aged muscles. The right gastrocnemius, plantaris, and soleus muscles from young adult (8 mo old) and late middle-aged (28-30 mo old) Fischer 344 x Brown Norway F1 hybrid rats were excised for evaluation of flux through electron transport chain complexes I-III and/or morphometric estimation of capillarization. Muscle mass was lower in the gastrocnemius muscles of the older animals (2,076 +/- 32 vs. 1,825 +/- 47 mg in young adult vs. late middle-aged, respectively; mean +/- SE) but not the plantaris or soleus muscles. Fibers were smaller in the white region of gastrocnemius muscles but larger in the red region of gastrocnemius muscles of the older animals. There was no difference in the number of capillaries around a fiber, the individual C/F ratio, or the CFPE index between groups for any muscle/region, whereas flux through complexes I-III was reduced by 29-43% in late middle-aged animals. Thus the greater quotient of indexes of anatomic capillarity (individual C/F ratio or CFPE index) and fiber oxidative capacity in soleus and the white region of gastrocnemius muscles, but not in the red region of gastrocnemius muscles of the older animals, shows that anatomic capillarity is maintained in relative excess of oxidative capacity in some muscle regions in late middle-aged rats.