Freezing at −20 °C and subsequent thawing of muscles from sheep, hare and deer, and of breast and leg muscle from chicken and duck, did not result in significant changes of the total activity of the enzymes citrate synthase and β-hydroxy-acyl-CoA-dehydrogenase; lipoamide dehydrogenase seemed to be somewhat more labile. Freezing and thawing of muscle tissue caused a partial release of these three mitochondrial enzymes into the sarcoplasmic fluid which indicates similar freeze-damage of the inner membrane of the mitochondria to that observed with bovine and porcine muscles.