To evaluate the "threshold" duration of exercise required to produce training effects, 18 healthy men aged 51 +/- 6 years completing 30 minutes of exercise training/day were compared with 18 men aged 52 +/- 6 years completing three 10-minute bouts of exercise/day, each separated by at least 4 hours. Exercise training intensity was moderate (65 to 75% of peak treadmill heart rate). During the 8-week study period VO2 max increased significantly in both groups from 33.3 +/- 3.2 to 37.9 +/- 3.5 ml/kg/min in men performing long exercise bouts and from 32.1 +/- 4.6 to 34.5 +/- 4.5 ml/kg/min in men performing short exercise bouts (p less than 0.05 within and between groups). Adherence to unsupervised exercise training performed at home and at work by men in long and short bouts was high; total duration of training completed was 96 and 93% of the prescribed amount and total number of sessions completed was 92 and 93% of that prescribed, respectively. In both groups exercise heart rate measured by a portable microprocessor was within or above the prescribed range for greater than 85% of the prescribed duration. Thus, multiple short bouts of moderate-intensity exercise training significantly increase peak oxygen uptake. For many individuals short bouts of exercise training may fit better into a busy schedule than a single long bout.