OBJECTIVE We propose a method to estimate O2 deficit (DefO2) during supramaximal exercise based on the analysis of gas exchanges. METHODS We determined in 14 male subjects breath-by-breath ∙VO2 at the onset of step exercise of moderate and supramaximal intensities. ∙VO2 response during moderate-intensity exercise was fitted as a bi-exponential model. Third, we modelled a theoretical ∙VO2 kinetics during supramaximal exercise assuming that it conformed to the ∙VO2 kinetics measured during moderate-intensity exercise and tended towards a ∙VO2 equal to that hypothetically elicited by the workload. The difference between the O2 volume that would have been consumed in the hypothetical condition and that actually taken up during exercise yielded the lactacid fraction of DefO2. Finally, the difference between the estimated total energy requirement and the theoretical O2 volume consumed by the subject yielded in the alactacid fraction of total DefO2. RESULTS Total DefO2 was 77.6 mL O2 kg-1; its anaerobic alactic and lactic fractions amounted to 38.5 and 39.1 mL O2 kg-1, respectively. The latter did not differ significantly from the value calculated independently from the peak increase in lactate concentration during supramaximal exercise times the energy equivalent of lactate. CONCLUSIONS These results suggest that supramaximal DefO2 and its components may be estimated from an analysis of gas exchange kinetics. In addition, they are consistent with the view that the mechanism regulating muscular O2 uptake is regulated by the muscular concentration of putative metabolites.