The short-term effects of cortisol and tyrosine on the hepatic enzymes phenylalanine hydroxylase and tyrosine aminotransferase were studied in both normal and adrenalectomized adult rats. Four hours following cortisol injection, phenylalanine hydroxylase activity was 13.1% of control in normal animals, but 288% of control in adrenalectomized animals. Tyrosine had no significant effect on the normal animals, but increased phenylalanine hydroxylase activity to 212% of control in adrenalectomized animals. Unidentified stimulating and inhibitory factors of the adrenal glands are postulated to account for these result.