Regardless of respiratory substrates, the relationship between the rates of ATP synthesis and oxygen consumption in skeletal muscle mitochondria working at different steady-state rates of ATP production were linear and parallel. Indeed, the slopes were not significantly different between re-fed and fasted mitochondria (ATP/O values for pyruvate were 2.03±0.13 and 2.52±0.22 in re-fed and fasted chicks, respectively; ATP/O values for palmitoyl-l-carnitine were 1.97±0.13 and 2.12±0.16 in re-fed and fasted chicks, respectively). It is important to understand here that the slope values describe the amount of extra oxygen that mitochondria have to consume to sustain an additional ATP production imposed by a change in the activity of the phosphorylating system. As such, the slope values would be close to the maximum overall stoichiometry (mechanistic ATP/O ratio) of mitochondrial oxidative phosphorylation and would mainly depend on the activity and coupling stoichiometry of oxidative phosphorylation complexes and the nature of the oxidized substrate. The above results therefore suggest that the mechanistic ATP/O did not differ between fasted and re-fed chicks.