In endotherms, regulation of the degree of mitochondrial coupling affects cell metabolic efficiency. Thus it may be a key contributor to minimising metabolic rate during long periods of fasting. The aim of the present study was to investigate whether variation in mitochondrial avian uncoupling proteins (avUCP), as putative regulators of mitochondrial oxidative phosphorylation, may contribute to ... the ability of king penguins (Aptenodytes patagonicus) to withstand fasting for several weeks. After 20 days of fasting, king penguins showed a reduced rate of whole animal oxygen consumption (-33%) at rest together with a reduced abundance of avUCP and peroxisome proliferator-activated receptor #61543; coactivator-1 #61537 & #61472 & #61480; PGC1- #61537; #61481; mRNA in pectoralis muscle (-54%, -36% respectively). These parameters were restored after the birds had been re-fed for 3 days. Further, in recently fed, but not in fasted, penguins, isolated muscle mitochondria showed a GDP-inhibited, fatty acid plus superoxide-activated respiration, indicating the presence of a functional UCP. It was calculated that variation in mitochondrial UCP-dependent respiration in vitro may contribute to nearly 20% of the difference in resting between fed or re-fed penguins and fasted penguins measured in vivo. These results suggest that the lowering of avUCP activity during periods of long-term energetic restriction may contribute to the reduction in metabolic rate and hence the ability of king penguins to face prolonged periods of fasting.