Propylthiouracil depressed and thyroxine elevated the succinoxidase activity of rat liver after 4 days of regeneration. Protein-depleted animals had a significant decrease in activity of succinic dehydrogenase and cytochrome oxidase, rate of restoration of liver mass, enzyme activity and new protein. Thyroxine increased the rate of regeneration and the enzyme activity even in protein-depleted animals. Adrenalectomized rats on low protein intake had a very low rate of regeneration and protein replacement and low amounts of new enzyme activity and new protein after partial hepatectomy. Increasing the dietary protein to 36% increased all of these factors significantly, but administration of cortisone or adrenal cortical extract, had little effect on enzyme activity even though its effect on protein restoration was about as good as that of the high protein intake. Apparently under certain conditions, these hormones can produce changes in specific enzyme proteins that are independent of changes in total cell protein.