The Eastern red spotted newt is an ectotherm that is active in winter and acclimation or acclimatization from summer to winter conditions results in behavioral, biochemical and metabolic modifications. Cytochrome c oxidase (CCO) is embedded in the inner mitochondrial membrane and in skeletal muscle, its activity increases in winter. The activity of citrate synthase (CS), a mitochondrial matrix enzyme, also increases in winter. The present study had two main purposes: 1) to include liver in our analysis; and 2) to determine the mechanisms by which the changes in enzyme activity are accomplished. To address goal 1 we measured the activity of CCO, CS and lactate dehydrogenase in liver tissue from summer- and winter-acclimatized newts. To address goal 2, in liver and skeletal muscle of summer- and winter-acclimatized newts we a) determined mitochondrial volume density and cristae surface area via TEM; b) measured the expression of genes encoding CCO and CS via rt-PCR; c) determined membrane fatty acid composition via gas chromatography of fatty acid methyl esters. We found: 1) CCO activity was higher in summer than winter in liver; 2) mitochondrial volume density decreased in muscle in winter while cristae surface area increased in winter in liver; 3) CCO activity was correlated with changes in membrane composition, not gross mitochondrial characteristics or gene expression; 4) CCO activity in liver did not correlate well with the parameters we measured; 5) direct quantification of mitochondrial properties by TEM is important.