Excess post-exercise oxygen consumption (EPOC) is normally not considered in determinations of the metabolic cost of activity. This approach overlooks an important energetic cost that an animal incurs as a result of activity. To examine the importance of EPOC, we determined how the energetic cost of locomotion was affected by activity of short duration and high intensity. Mice were run at maximum speed on a treadmill while enclosed in an open-flow respirometry system. After sprinting for 5, 15, 30, or 60 sec, each mouse was allowed to recover while remaining enclosed in the respirometry chamber. Exercise oxygen consumption (EOC), the volume of oxygen consumed during the exercise, increased linearly with sprint duration. EPOC was determined as the volume of oxygen consumed after exercise ended until rest was reached. EPOC volumes were found to be constant following 5-60 sec of activity and accounted for > or = 90% of the total metabolic cost. The average EPOC volume of all treatments was 0.76 +/- 0.456 ml O2.gm-1. The net cost of activity (Cact), which considers both EOC and EPOC, decreased as sprint duration increased and varied between 500 ml O2.g-1.km-1 for 5 sec to 30 ml O2.g-1.km-1 for 60 sec of activity. The values for Cact were 15 to 250 times higher than traditional estimates of locomotor costs. From these data, it can be concluded that (1) EPOC is not affected by short exercise durations; (2) EPOC is an important energetic consideration when exercise durations are short; and (3) the metabolic costs of brief, vigorous locomotion may be much higher than previously estimated.