This study examined the possible effects of caffeine ingestion on muscle metabolism and endurance during brief intense exercise. We tested 14 subjects after they ingested placebo or caffeine (6 mg/kg) with an exercise protocol in which they cycled for 2 min, rested 6 min, cycled 2 min, rested 6 min, and then cycled to voluntary exhaustion. In each exercise the intensity required the subject's maximal O2 consumption. Eight subjects had muscle and venous blood samples taken before and after each exercise period. The caffeine ingestion resulted in a significant increase in endurance (4.12 +/- 0.36 and 4.93 +/- 0.60 min for placebo and caffeine, respectively) and resulted in a significant increase in plasma epinephrine concentration throughout the protocol but not in norepinephrine concentration. During the first two exercise bouts, the power and work output were not different; blood lactate concentrations were not affected significantly by caffeine ingestion, but during the exercise bouts muscle lactate concentration was significantly increased by caffeine. The net decrease in muscle glycogen was not different between treatments at any point in the protocol, and even at the time of fatigue there was at least 50% of the original glycogen concentration remaining. the data demonstrated that caffeine ingestion can be an effective ergogenic aid for exercise that is as brief as 4-6 min. However, the mechanism is not associated with muscle glycogen sparing. It is possible that caffeine is exerting actions directly on the active muscle and/or the neural processes that are involved in the activity.