Nine subjects participated in an exercise program for 40 min/day, 6 days/wk. The training work rates were kept constant for the first 4 wk. The work rates were then increased to a higher level and kept constant for an additional 5 wk. During both training periods, maximum oxygen uptake (VO2max) increased for the first 3 wk and then remained constant. The half times (tl/2) of the increases in VO2max during the two periods were 10.3 and 10.8 days. These results provide evidence that the adaptation to endurance exercise of the system(s) that limit VO2max is rapid, with a tl/2 of less than 11 days. The total increase in VO2max in the 9 wk of training was 23%. The decreases in the heart rate and blood lactate responses to a standard submaximal exercise test also occurred within the first 2 to 3 wk of each training period. Our results show that unless the training stimulus is increased, a high intensity daily exercise program does not result in a further increase in VO2max or further decreases in the blood lactate or heart rate responses to submaximal exercise after 3 wk.