The time-course of adaptation in cardiorespiratory fitness, measures of capillarization, and citrate synthase (CS) activity were examined in seven older (O; 69 ± 7 years) and seven young (Y; 22 ± 1 years) men pre-, mid-, and posttraining during a 12-week endurance training program. Training was performed on a cycle ergometer three times per week for 45 minutes at ~70% of maximal VO(2) (VO(2max)). VO(2max) and maximal cardiac output increased similarly from pre- to posttraining in O and Y (p < .05), and maximal a-vO(2diff) was greater (p < .05) posttraining in O and Y. CS was elevated at mid- and posttraining compared with pretraining in both O and Y (p < .05). Indices of capillarization increased 30%-40% in O and 20%-30% in Y and were elevated at posttraining compared with pre- and midtraining in both groups (p < .05). This study showed that both O and Y undertaking similar endurance training displayed capillary angiogenesis and improved mitochondrial respiratory capacity.