The purpose of this study was to obtain information regarding the minimum training frequency necessary to maintain the exercise-induced increase in maximum oxygen uptake (VO2max). Twelve subjects (average age = 23 yr) participated in an endurance exercise program of bicycling and running 40 min/d, 6 d/wk. After 10 wk, they continued to train either 4 d/wk or 2 d/wk for an additional 15 wk. Intensity and duration for the additional 15 wk remained the same as on the tenth week of training. The average increase of VO2max in response to 10 wk of training was 25% when measured during bicycle testing and 20% when measured during treadmill testing. VO2max in the 4 d/wk and 2 d/wk groups remained essentially the same as the trained levels when determined at 5-wk intervals of reduced training. These results provide evidence that more exercise is required to increase VO2max, than that required to maintain it at the trained level in young adults performing high-intensity exercise. Furthermore, it is possible to maintain the increased VO2max for at least 15 wk by training at high intensity for 2 d/wk or 4 d/wk.