As body temperature decreases, changes in the physical chemistry of the cell produce a reduction in metabolic activity. In temperate fish, cold water temperatures either lead to dormancy or else trigger a range of homeostatic responses which serve to offset the passive effects of reduced temperature. Compensatory adjustments to temperature occur with time courses ranging from less than a second to more than a month. Although swimming performance may increase with cold-acclimation, active metabolic rate remains significantly below that for warm-acclimated fish. Compensatory and dormancy responses are not mutually exclusive and sometimes occur in the same species depending on the temperature. Cold-acclimation results in significant increases in the density of mitochondria and capillaries in skeletal muscle. This serves to reduce diffusion distances and increase the capacity for aerobic ATP production relative to fish acutely exposed to low temperature. There is evidence that cold acclimation has differential effects on the synthesis and degradation rates of mitochondrial proteins leading to a net increase in their concentration. In contrast, the activities of enzymes associated with glycolysis and phosphocreatine hydrolysis show no consistent changes with thermal acclimation suggesting that flux through these pathways is modulated by factors other than enzyme concentration. Higher mitochondrial densities have also been reported for the liver, brain and gill tissue of cold compared with warm acclimated fish. In spite of their increased concentration, the activities of aerobic enzymes remain much lower at cold than warm temperatures. Acclimation temperature affects hepatosomatic index, the concentration of energy reserves, and the relative importance of glucose and fatty acid catabolism in liver. The fraction of glucose oxidized by the hexose monophosphate shunt (HMPS) pathway also increases with cold acclimation in some species. It is likely that many of the changes in liver metabolism with temperature acclimation reflect associated changes in feeding behaviour and/or diet, and other energetic demands (e.g. gametogenesis). Possible mechanisms underlying alterations in pathway utilization with temperature acclimation are discussed. They include changes in factors influencing enzyme structure and activity (e.g. pH, substrate/modulator concentrations, phosphorylation state, membrane composition), and effects of temperature on gene expression.