The study of the kinetics of O₂ consumption (V̇o₂) at the onset and offset of constant-load submaximal exercise (V̇o₂ on- and off-kinetics) is useful from a practical point of view (a faster adjustment of oxidative metabolism following an increased metabolic demand reduces the need for substrate level phosphorylation, with implications on exercise tolerance and muscle fatigue) and can give valuable insights into the regulation of oxidative metabolism in skeletal muscle. Measurements have been carried out both in man and in animals, at the tissue and at the whole body level. At the tissue level, the V̇o₂ on- and off-kinetics were determined: a) Directly, by dynamic solution of the Fick equation throughout the transients; attempts were also made to obtain similar informations by near-infrared spectroscopy. b) Indirectly, from the kinetics of phosphocreatine hydrolysis and resynthesis, by chemical methods or by ³¹P magnetic resonance spectroscopy. At the whole body level, V̇o₂ on- and off-kinetics are determined from breath-by-breath measurements of pulmonary gas exchange. The V̇o₂ = f(t) function is a complex one, particularly during the on-transient. The so-called “phase 2” of the V̇o₂ on-response, as well as the V̇o₂ off-response, yield relevant metabolic informations. In muscle the V̇o₂ on- and off-kinetics are characterized by half-times (t½) of 15–20 sec. At the whole-body level, t½ of the V̇o₂ on-kinetics show a wider variability, related to the experimental protocol and to other factors. The V̇o₂ off-phase is more constant, and its kinetic parameters appear closer to those obtained at the tissue level. The study of the V̇o₂ kinetics is valuable for a functional evaluation of skeletal muscle oxidative metabolism. In ordinary conditions muscle V̇o₂ kinetics appears mainly imposed by intrinsic (metabolic) rather than extrinsic (O₂ delivery) factors.