The relationships between the plasma epinephrine threshold (TE), the norepinephrine threshold (TNE), and the blood lactate threshold (Tlact) were examined during incremental cycling in 10 untrained male subjects. When oxygen uptake (VO2) measured at each threshold was expressed as a percent of VO2max, the thresholds occurred at 40.8 +/- 2.4%, 45.5 +/- 3.0%, and 46.6 +/- 3.0% for Tlact, TE, and TNE, respectively. The average VO2 and power output values obtained at the lactate and epinephrine thresholds were not found to be significantly different (p < 0.10). However, Tlact and TE occurred simultaneously in six subjects, whereas TE occurred at a higher work stage than Tlact in the other four subjects. The mean VO2 and power output values determined at TNE were found to be significantly greater than the values obtained at Tlact (p < 0.05). These two thresholds occurred together in four subjects, while TNE was observed to occur at a higher work stage than Tlact in the other six subjects. None of the differences between TE and TNE were significantly different. Although plasma norepinephrine concentrations were much greater than those for epinephrine at a given power output, the two catecholamine thresholds occurred simultaneously in seven subjects. The results are not consistent with the hypothesis that the increase in plasma catecholamine levels during incremental exercise is the sole determinant of the lactate threshold. It is also possible that a decrease in muscle pH, due to increased lactic acid, stimulated a reflex increase in sympathetic outflow and a subsequent rise in catecholamine levels.