To examine the metabolic and hormonal responses to non-exhaustive steady-state exercise at the individual anaerobic threshold (IAT), 12 male physical education students performed treadmill exercise of 50 min duration. The treadmill speed equaled that at the IAT as assessed in a standardized progressive exercise test (75 +/- 2% of maximal oxygen uptake). Heart rate averaged 177.0 +/- 12.2 min-1 at 15 min and 184.5 +/- 11.5 min-1 at 50 min. After the initial adjustment, arterial lactate stabilized at individually different levels between 2.70 and 6.00 mmol/l without any substantial trend in the individual curves. Arterial glucose was unchanged throughout the test. Glycerol increased continuously to 157% above the preexercise value (P less than 0.001). The FFA blood level was not depressed but rather showed an increasing tendency between 25 and 50 min (P less than 0.05). Between 0 and 25 min, insulin decreased (P less than 0.01), growth hormone increased to 8 times its pre-exercise value (P less than 0.001), and cortisol did not show any significant changes. Between 25 and 50 min, no significant additional changes were detected for these hormones. At 15 min epinephrine and norepinephrine had increased 2.8- and 7-fold above the respective pre-exercise values (P less than 0.001); both catecholamines continued to increase until 50 min (P less than 0.001 and P less than 0.01). It is concluded that prolonged exercise at the IAT is associated with a steady-state condition in carbohydrate supply and turnover, as is suggested by the stable blood levels of glucose and lactate. The stably elevated blood level of lactate did not result in depression of the FFA blood level, suggesting unimpaired supply of FFA from extramuscular sources. Exercise at the IAT places a high load on aerobic metabolism without encountering progressive lactate accumulation and the associated metabolic effects.