The effects of prolonged endurance training on maximal O2 uptake capacity (VO2max) and its determinants were studied in 11 older individuals (63 +/- 2 yr). The subjects were evaluated before training, after 6 mo of low-intensity (LI) training, and after an additional 6 mo of higher intensity (HI) training. VO2max was 25.4 +/- 4.6 ml X kg-1 X min-1 before training, 28.2 +/- 5.2 ml X kg-1 X min-1 after LI training (P less than 0.05), and 32.9 +/- 7.6 ml X kg-1 X min-1 after HI training (P less than 0.01), with an overall increase of 30%. The increase in VO2max in response to training appeared to be mediated primarily through an increase in maximal arteriovenous O2 difference (P less than 0.01), with little augmentation of maximal cardiac output (Q) (P greater than 0.05). At the same absolute work rates, stroke volume was higher (P less than 0.05); heart rate (HR), blood pressure (BP), and systemic vascular resistance were lower (P less than 0.05); and Q and arteriovenous O2 difference were unchanged after training. At the same relative work rates, arteriovenous O2 difference was higher (P less than 0.01); BP and systemic vascular resistance were lower (P less than 0.05); and HR, Q, and stroke volume did not change significantly. These findings show that older individuals can adapt to prolonged endurance training with a large increase in aerobic power.