Capacities and effects of cold or warm acclimation were investigated in two zoarcid species from the North Sea (Zoarces viviparus) and the Antarctic (Pachycara brachycephalum) by investigating temperature dependent mitochondrial respiration and activities of citrate synthase (CS) and NADP+ -dependent isocitrate dehydrogenase (IDH) in the liver. Antarctic eelpout were acclimated to 5°C and 0°C (controls) for at least 10 months, whereas boreal eelpout, Z. viviparus (North Sea) were acclimated to 5°C and to 10°C (controls). Liver sizes were found to be increased in both species in the cold, with a concomitant rise in liver mitochondrial protein content. As a result, total liver state III rates were elevated in both cold-versus and warm-exposed P. brachycephalum and Z. viviparus, with the highest rates in boreal eelpout acclimated to 5°C. CS and IDH activities in the total liver were similar in Z. viviparus acclimated to 5°C and 10°C, but decreased in those warm acclimated versus control P. brachycephalum. Enzyme capacities in the total liver were higher in eelpout from Antarctica than those from the North Sea. In conclusion, cold compensation of aerobic capacities in the liver seems to be linked to an increase in organ size with unchanged specific mitochondrial protein content. Despite its life in permanently cold climate, P. brachycephalum was able to reduce liver aerobic capacities in warm climate and thus, displayed a capacity for temperature acclimation. © Springer-Verlag 2005.