Both aerobic training (AT) and resistance training (RT) may increase aerobic power (VO2peak) in the older population; however, the role of changes in the capillary supply in this response has not been evaluated. Twenty healthy men (age 65-74 yr) engaged in either 9 wk of lower body RT followed by 9 wk of AT on a cycle ergometer (RT-->AT group) or 18 wk of AT on a cycle ergometer (AT-->AT group). RT was performed three times per week and consisted of three sets of four exercises at 6-12 repetitions maximum. AT was performed three times per week for 30 min at 60-70% heart rate reserve. VO2peak was increased after both RT and AT (P < 0.05). Biopsies (vastus lateralis) revealed that the number of capillaries per fiber perimeter length was increased after both AT and RT (P < 0.05), paralleling the changes in VO2peak, whereas capillary density was increased only after AT (P < 0.01). These results, and the finding of a significant correlation between the change in capillary supply and VO2peak (r = 0.52), suggest the possibility that similar mechanisms may be involved in the increase of VO2peak after high-intensity RT and AT in the older population.