This paper: 1) Reviews evidence for the location of the slow component of &OV0312;O2kinetics either within the exercising limbs or alternatively at some site in the rest of the body, e.g., ventilatory, cardiac or accessory muscles. 2) Presents evidence in support of both the fast and slow components (i.e., <3 min and >3 min from exercise onset, respectively) of the exercise &OV0312;O2response residing predominantly in the exercising muscle. For a pulmonary &OV0312;O2slow component in excess of 600 ml O2·min−1, more than 80% could be attributed to an augmented &OV0312;O2across the exercising limbs. 3) Assesses the potential for the lactate ionper seto exert a metabolic stimulatory effect in exercising muscle in the absence of the potentially confounding influences of changes in muscle temperature, H+, blood flow or O2delivery. Within the surgically isolated, electrically stimulated canine gastrocnemius, square wave infusions that increased arterial blood [lactate] by ∼10 mM and intramuscular [lactate] to in excess of 9 mM did not increase muscle &OV0312;O2. In summary, these investigations demonstrate that the exercising muscle is the predominant site of the &OV0312;O2slow component. However, despite the close temporal association between changes in blood lactate and &OV0312;O2during intense exercise, lactate itself does not mandate an additional &OV0312;O2demand in exercising dog muscle.