Abstract 1. 1. Chicken liver mitochondria were isolated in relatively pure form as indicated by electron microscopy and marker enzyme assay. 2. 2. The rate of respiration, respiratory control index and ADP/O ratios with several different substrates indicated that chicken liver mitochondria are more uncoupled than rat liver mitochondria. 3. 3. Chickens have ten-fold higher malate concentrations in liver than do rats, 2-oxoglutarate was also more abundant in chicken livers. 4. 4. Fasted birds had a five-fold increase in β-hydroxybutyrate as compared with fed birds; whereas malate and lactate concentrations decreased. 5. 5. Fasted birds had increased levels of isocitrate dehydrogenase (NADP dependent) and lactate dehydrogenase in the cytosol, and increased malate dehydrogenase (NAD dependent), isocitrate dehydrogenase (NADP dependent) and malic enzyme activities in the mitochondria.