The main purpose of this study was to present a new method to determine the level of power output (PO) at which VCO2 during incremental exercise test (IT) begins to rise non-linearly in relation to power output (PO) - the change point in VCO2 (CP-VCO2). Twenty-two healthy non-smoking men (mean +/- SD: age 22.0 +/- 0.9 years; body mass 74.5 +/- 7.5 kg; height 181 +/- 7 cm; VO2max 3.753 +/- 0.335 l min-1) performed an IT on a cycloergometer. The IT started at a PO of 30 W, followed by gradual increases of 30 W every 3 min. Antecubital venous blood samples were taken at the end of each step and analysed for plasma lactate concentration [La]pl, blood PO2, PCO2 [HCO3-]b and [H+]b. In the detection of the change-point VCO2 (CP-VCO2), a two-phase model was assumed for the 'third-minute-data' of each step of the test. In the first phase, a linear relationship between VCO2 and PO was assumed, whereas in the second, an additional increase in VCO2 was allowed, above the values expected from the linear model. The PO at which the first phase ends is called the change point in VCO2. The identification of the model consists of two steps: testing for the existence of the change point, and estimating its location. Both procedures are based on suitably normalized recursive residuals (see Zoladz et al. 1998a. Eur J Appl Physiol 78, 369-377). In the case of each of our subjects it was possible to detect the CP-VCO2 and the CP-VO2 as described in our model. The PO at the CP-VCO2 amounted to 134 +/- 42 W. The CP- VO2 was detected at 136 +/- 32 W, whereas the PO at the LT amounted to 128 +/- 30 W and corresponded to 49 +/- 11, 49 +/- 8 and 47 +/- 8.6% VO2max, respectively, for the CP-VCO2, CP-VO2 and the LT. The [La]pl at the CP-VCO2 (2.65 +/- 0.76 mmol L-1), at the CP-VO2 (2.53 +/- 0. 56 mmol L-1) and at the LT (2.25 +/- 0.49 mmol L-1) were already significantly higher (P < 0.01, Students t-test) than the value reached at rest (1.86 +/- 0.43 mmol L-1). Our study illustrates that the CP-VCO2 and the CP-VO2 occur at a very similar power output as the LT. We therefore postulate that the CP-VCO2 and the CP-VO2 be applied as an additional criterion to assess human exercise tolerance.