The O2 path from environment to mitochondria can be viewed as a cascade of resistances in series, each being overcome by a specific pressure gradient (O2 conductance equation). To assess the relative importance of the different factors that can set a limit to VO2 max, three sets of resistances will be identified, RQ, Rc and Rm, inversely proportional to: O2 transport (Qmax X [Hb]), RQ; capillary cross section, Rc; and succinic dehydrogenase (SDH) activity, Rm. Published data show that changes of VO2 max can be induced by altering the blood O2 capacity, or by training, and that these changes are accompanied by measured changes of the above identified resistances. From these data, the ratio of each resistance to the overall resistance can be calculated by algebraic manipulation of the O2 conductance equation, expressed in relative form. It can thus be shown that: in two-legged exercise, about 75% of VO2 max is set by O2 transport, the remaining fraction being about equally partitioned between the two peripheral factors indicated above, and that in one-legged exercise, the limits to VO2 max are about equally set by central and peripheral factors.