The effect of a 40-week training program on the anaerobic threshold (AT) was studied in five subjects (35 +/- 5 yrs). The training program consisted of a bicycle ergometer exercise 1 h per day 3 days a week at a work load corresponding to 80%-85% of VO2 max. Before training (S0) and at the 10th, 20th, 30th, 40th weeks (S10, S20, S30, S40) of the training program, ventilatory AT (AT vent), lactate AT (AT lact), and 4 mmol AT were estimated using a graded exercise test. In the same period, another test (1) to determine VO2 max, maximal work load (MWL), and net efficiency (2) and a 1-h endurance exercise requiring 85%-90% of VO2 max were performed. After training, AT increased significantly by 10%, 11%, and 18% in AT vent, AT lact, and AT 4 mmol, respectively. MWL and net efficiency increased by 22% and 14% respectively, and there was no significant increase in VO2 max. The AT increase at S20 was delayed as compared to the change in MWL (S10). It is hypothesized that an AT increase provides good evidence for modifications of the muscular oxidative capacities during an endurance training program. A part of this paper is devoted to (1) the study of the reproducibility of AT estimation, (2) a comparison to other methods for determining the definition of AT, and (3) the correlations between the three methods utilized for AT estimation.