Summary. Rat livers perfused with whole rat blood and added amino acids support the induction of tryptophan oxygenase and tyrosine aminotransferase by hydrocortisone to about half the levels reached after 5 h in vivo. The added amino acids were necessary for the induction of tryptophan oxygenase. In addition, the tryptophan oxygenase was induced by the substrate analogue, α-methyltryptophan. Glucagon induced tyrosine aminotransferase, but not tryptophan oxygenase, in livers isolated from adrenalectomized rats, as it does in vivo.