In a recent review (Dewhurst, El Kabir, Harris and Mandelbrote, 1968) the evidence relating the effect of stress to thyroid function has been surveyed. In that account the comment was made that although it is impossible to assess or quantify the psychological trauma that any mentally ill patient is undergoing, it would seem reasonable to assume that if emotional stress is associated with abnormal thyroid activity, then a study of thyroid function in mentally ill patients would reveal such abnormal activity. In the present work a study has been made of the function of the pituitary–thyroid axis in control subjects and in patients suffering from schizophrenia and affective illnesses. Since there is strong evidence that emotional stress in many cases affects endocrine function through the mediation of the hypothalamus and anterior pituitary gland, measurement of the blood concentration of thyrotrophic hormone (T.S.H.) has been used in the present investigation. A further reason for assaying the blood concentration of T.S.H. is that measurement of many of the more direct parameters of thyroid function are subject to variables which are difficult to control. Studies of the blood level of T.S.H. in psychiatric patients have not, so far as we know, been carried out previously.