The basal non-phosphorylating respiration rates measured in the presence of oligomycin (the intercepts with the x-axis) were significantly higher in re-fed than in fasted mitochondria respiring on pyruvate (+44%) or palmitoyl-l-carnitine (+60%). Hence, regardless of the substrate used, the linear relations concerning the re-fed chicks were significantly shifted to the right compared with fasted birds, indicating lower oxidative phosphorylation efficiency (effective ATP/O) because, to produce a given amount of ATP, mitochondria consumed more oxygen in re-fed than in fasted chicks. This is shown more clearly when effective ATP/O ratios are plotted against the oxygen consumption rate (Fig. 2C for pyruvate/malate; Fig. 2D for palmitoyl-l-carnitine/malate). In the range of oxygen consumption measured, the effective ATP/O was higher in fasted than in re-fed chicks, irrespective of the substrate used. Contrary to the mechanistic ATP/O (slope values as described above), the effective ATP/O would be affected by the activities of mitochondrial reactions that consume the proton gradient built up by the respiratory chain, mainly proton leak reactions and the phosphorylation system (ATP synthase, phosphate carrier and adenine nucleotide translocase), which compete for the same driving force (Brand et al., 1993). Altogether, these results therefore suggest that mitochondrial efficiency had been altered by extrinsic properties of oxidative phosphorylation machinery. On the whole, the effective oxidative phosphorylation efficiency of skeletal muscle mitochondria was lowered by an average of 15% in re-fed animals (Fig. 4).