Dark-cutting beef was produced in heifers by subcutaneous injection of epinephrine. Muscle samples, removed at slaughter, were examined to assess the effects of the hormone on oxidative metabolism. Mitochondria were isolated and examined polarographically, and for succinic dehydrogenase. No significant differences were found in mitochondrial protein yield, mitochondrial function, or in specific activity of succinic dehydrogenase between preparations from control and experimental animals. In addition, the specific activities of Phosphorylase and a-glycerol phosphate dehydrogenase were not significantly different in the two groups. Measurements of active (a) and inactive (b) Phosphorylase confirmed that a higher percentage of active Phosphorylase was present in muscles of epinephrine treated animals.