The purpose of this study is to investigate the effect of work load durations on the relationships between aerobic and anaerobic thresholds determined from gas exchange parameters, and lactate thresholds determined from blood lactate concentrations. Aerobic and anaerobic thresholds (Aer T, An T) were detected from the plot of changes in gas exchange parameters against increasing work load by a breath-by-breath system. Lactate thresholds (LT 1, LT 2) were detected from the plot of blood lactate level in the same manner as Aer T and An T. Ten normal subjects were studied doing progressive bicycle ergometer exercise (after 2 minutes warming-up at 0.5kp) until exhaustion under three experimental conditions (work load duration was increased every 1, 3 and 5 minute by 0.5kp respectively) in a random order. Neither aerobic nor anaerobic threshold expressed in VO₂ was significantly different despite of the different work load durations, but those data expressed as work load (kp) had significant differences with the different work load durations (for both Aer T and An T, p0.01). Significant correlations were found between the average values of VO₂ at Aer T and LT I (for 3 min., p0.05, for 5 min., p0.01), and between An T and LT 2 (for all of 1, 3 and 5 min., p 0.01). These results suggest that Aer T and An T were well coincide with LT 1 and 2 under any experimental conditions studied here. Therefore, it seems that Aer T and An T determined from gas exchange parameters can be used as an index of the aerobic performance in man.