In order to determine the effects of caffeine ingestion on performance and metabolic responses during supramaximal exercise, six healthy volunteers performed the Wingate Anaerobic Test twice. Sixty min before each trial, while in a fasting state, they took capsules containing either caffeine (5 mg/kg) or a placebo, according to a single blind and randomized procedure. Caffeine administration did not significantly change either maximal anaerobic capacity (AC) or power (AP) and power decrease (PD). It did, however, induce significant (p less than 0.05) increases in both catecholamine and blood lactate levels as compared to values obtained after placebo administration. Moreover, maximal blood lactate occurred earlier (p less than 0.05), and lactate output seemed to be greater with caffeine (p less than 0.01). There was a strong correlation, both with and without caffeine, between epinephrine and lactate levels (r = 0.81) and between both AP and AC and lactate levels. These data suggest that caffeine, essentially via epinephrine, modifies glycolytic metabolism but fails to improve performance during the Wingate Anaerobic Test in nonspecifically trained subjects.