Five male subjects performed steady exercise on a cycle ergometer at an intensity of 60% maximal O2 uptake (VO2max) for 6 min on three separate occasions while breathing gas mixtures of 12,16 or 21% O2 in N2. Expired gas fractions, ventilation, heart rate, arterial O2 saturation (SaO2), blood lactate (La) and plasma catecholamines (epinephrine: E and norepinephrine: NE) were measured. O2 uptake (VO2) was calculated for the last minute of exercise. Blood samples were drawn at rest and immediately after exercise. By inspiring hypoxic gas mixtures, the SaO2 value decreased during exercise to 85.0 +/- 5.4 (16%) and 66.4 +/- 4.1 (12%) from 95.0 +/- 0.1 in normoxia. VO2 during exercise was not different among the three conditions. Exercise-induced La accumulation was increased by hypoxia. E and NE during exercise were not affected by hypoxia statistically. There was a significant correlation between La and E (P < 0.01) and between La and NE (P < 0.01) during exercise in the three conditions. The present findings suggest a relationship between glycogen metabolism and sympathoadrenal activity which results in an increase of plasma catecholamines during exercise in humans acutely exposed to hypoxia.