Eight subjects exercised for 40 min/day, 6 days/wk for 10 wk. For 3 days/wk they performed six 5-min intervals of bicycling on an ergometer against a resistance that elicited VO2 max, separated by 2-min intervals of exercise requiring 50-60% of Vo2 max. On the alternate 3 days, they ran as far as they could in 40 min. Our purpose was to obtain information regarding the time course and magnitude of the increase in Vo2 max and endurance that occur in response to strenuous exercise when the training stimulus is kept approximately constant relative to maximum aerobic capacity. Average Vo2 max increased 5% (P less than 0.05) during the 1st wk. Endurance, Vo2 max, and time to attainment of peak heart rate all increased linearly during the 10 wk. The average weekly increase in Vo2 max was 0.12 l/min. The total increase in Vo2 max averaged 16.8 ml/kg per min (44%). Four of the eight subjects attained Vo2 max levels approaching or exceeding 60 ml/kg per min. It appears from these results that aerobic work capacity can increase more rapidly and to a greater extent in response to training than has generally been thought.