Theoretical best performance times (ttheor) in track running are calculated as follows. Maximal metabolic power (Emax) is a known function of maximal oxygen uptake (VO2max), of maximal anaerobic capacity (AnS) and of effort duration to exhaustion (te):Emax=f (te). Metabolic power requirement (Er) to cover the distance (d) in the performance time tp is the product of the energy cost of locomotion per unit distance (C) and the speed: Er=Cxd/tp. The time values for which Emax (te)=Er (tp), assumed to yield ttheor, can be obtained for any given subject and distance provided that VO2max, AnS and C are known, and compared with actual best performances (tact). For 15 min> or =te> or =100 s, the overall ratio tact/ttheor was rather close to 1.0. To estimate the relative role of the different factors limiting VO2max, several resistances to O2 transport are identified, inversely proportional to: alveolar ventilation (RV*), O2 transport by the circulation (RQ), O2 diffusion from capillary blood to mitochondria (Rt), mitochondrial capacity (Rm). Observed changes of VO2max are accompanied by measured changes of several resistances. The ratio of each resistance to the overall resistance can therefore be calculated by means of the O2 conductance equation. In exercise with large muscle groups (two legs), RQ is the major (75%) limiting factor downstream of the lung, its role being reduced to 50% during exercise with small muscle groups (one leg). Rt and Rm account for the remaining fractions. In normoxia RV* is negligible; at high altitude it increases progressively, together with Rt and Rm, at the expense of RQ.