Oxidative phosphorylation efficiency of mitochondria isolated from pectoralis muscle of fasted or re-fed chicks. Oxygen uptake and ATP synthesis rate were titrated by addition of increasing concentrations of ADP in the presence of glucose (20 mmol l−1), hexokinase (1.5 U ml−1) and either pyruvate/malate (A and C) or palmitoyl-l-carnitine/malate (B and D) at 38°C. Values are means±s.e.m. for N=11 (fasted, open symbols) and N=9 (fed, filled symbols) independent mitochondrial preparations.