Twenty-nine subjects trained for 6 weeks for the purpose of comparing the effects of various durations of exercise. All subjects trained three days per week at 75% of maximum heart rate. Training durations were 5 min (N = 10), 15 min (N = 10) and 25 min (N = 9) with over 80% of the training sessions consisting of monitored treadmill walking on a grade. Pre and post-testing consisted of a maximal treadmill walk and a submaximal bicycle ride for the three training groups and a control group (N = 10). The improvements in treadmill time were 7.7%, 14.7% and 20.2% for the three training groups, all significant. The improvement for the 5 min group was not different than that of the control or 15 min groups. Improvement in the 25 min group was significantly greater than the control and 5 min groups, but not the 15 min group. The same differences were noted when estimated maximal oxygen uptake changes were analyzed. The results indicate that at least 15 and preferably 25 min of exercise is necessary to produce optimal endurance improvement with the training intensity and frequency used in this study.