Prolonged physical exercise tests (50 min) at the threshold of 4 mmol . l-1 lactate (ATc) and at the individual anaerobic threshold (IAT) were applied in 19 rowing athletes. In each of the rowers (n = 19) work loads corresponding to the IAT did not result in a gradual lactase accumulation or exhaustion within 50 min of exercise. Means of lactate concentration and heart rate at the end of exercise were 4.0 +/- 1.6 mmol . l-1 and 182 +/- 13.0 beats . min-1, respectively. In 15 of 19 rowers, the IAT corresponded to lower work loads than the ATc. In these cases, prolonged exercise tests at the ATc showed gradual increases in lactate concentrations to a mean of 9.6 +/- 1.2 mmol . l-1, associated with exhaustion at a mean working time of 14.4 +/- 6.3 min and a mean heart rate of 192 +/- 10.4 beats . min-1. In four rowers, the IAT was found at identical (n = 3) or higher (n = 1) work loads than the ATc. In these cases, after an initial increase no further rise in lactate concentrations in blood was observed, and exhaustion did not occur during the prolonged exercise tests. These findings support the conclusion derived from the lactate kinetics model that the IAT defines the work load at the maximal lactate steady state.