Tyrosine and free thyroxine in the CSF and in serum has been measured in 24 euthyroid patients, in 11 patients with thyrotoxicosis and in 4 patients suffering from myxoedema. The thyrotoxic patients had elevated tyrosine levels in the CSF and in the serum, but rise in tyrosine was greater in the CSF than in the serum and the mean ratio CSF tyrosine/serum tyrosine was significantly elevated as compared to the euthyroid group. In the hypothyroid group, in spite of decreased serum tyrosine values the CSF tyrosine was not decreased. Free thyroxine in the CSF was elevated in the thyrotoxic patients and decreased in the hypothyroid patients. In euthyroid, hyperthyroid and hypothyroid patients an equilibrium was found between free thyroxine in the CSF and in the serum. No correlation was found within the groups between the individual values of tyrosine in the serum and in the CSF or between tyrosine and free thyroxine in the serum and in the CSF.