Factors contributing to maximal incremental and short-term exercise capacity were measured before and after 12 wk of high-intensity endurance training in 12 old (60-70 yr) and 10 young (20-30 yr) sedentary healthy males. Peak O2 uptake in incremental cycle ergometer exercise increased from 1.60 +/- 0.073 to 2.21 +/- 0.073 (SE) l/min (38% increase) in the old subjects and from 2.54 +/- 0.141 to 3.26 +/- 0.181 l/min (29%) in the young subjects. Peak cardiac output, estimated by extrapolation from a series of submaximal measurements by the CO2 rebreathing method, increased by 30% (from 12.7 to 16.5 l/min) in the old subjects, associated with a 6% increase (from 126 to 135 ml/l) in arteriovenous O2 difference; in the young subjects there were equal 14% increases in both variables (18.0 to 20.5 l/min and 140 to 159 ml/l, respectively). Submaximal mean arterial pressure and cardiac output were lower posttraining in the old subjects; total vascular conductance and cardiac stroke volume increased. Although peak power at the start of a short-term maximal isokinetic test did not change, total work accomplished in 30 s at a pedaling frequency of 110 revolutions/min increased in both groups, from 11.2 to 12.6 kJ and from 15.7 to 16.9 kJ in the old and young, respectively; fatigue during the 30-s test was less, and postexercise plasma lactate concentrations were lower. In older subjects, increases in aerobic power after high-intensity endurance training are at least as large as in younger subjects and are associated with increases in vascular conductance, maximal cardiac output, and stroke volume.