Resistance training (RT) has been shown to increase aerobic power in older humans. To determine the effects of RT on the capillary supply in this population, nine older men (65-74 y) engaged in 9 weeks RT of the lower body. Following RT, peak O2 uptake (V.O2,peak) increased by 7% (P<.01). Needle biopsies (vastus lateralis muscle) revealed significant increases (mean +/- SE) in fibre area (3,874 +/- 314 microm2 to 4,778 +/- 309 microm2), fibre perimeter (P, 262 +/- 11 microm to 296 +/- 11 microm), capillary contacts (3.7 +/- .2 to 4.3 +/- .3) and the individual capillary-to-fibre ratio (C:Fi, 1. 33 +/- .32 to 1.61 +/- .37, P<.005). To evaluate the potential for blood-tissue exchange, both fibre area-based and perimeter-based measures of the capillary supply were compared. While the area-based measures were maintained, C:Fi/P was increased, consistent with an increase in the size of the fibre-capillary interface and thus, an increased potential for oxygen flux following RT. Of the measurements of capillary supply, V.O2,peak correlated best with C:Fi/P (r = 0.69, P<.005). These results indicate a significant increase in the capillary supply relative to the perimeter, but not the cross-sectional area, of the muscle fibres following RT in older men, and that C:Fi/P is strongly correlated to the V.O2,peak in this population.