A thyrotropin (T.S.H.) test is suggested which relies on the change in the concentration of serum thyroxine as the index of thyroid response to T.S.H. stimulation. Ten such tests have been carried out in healthy volunteers and 39 in patients referred for routine thyroid stimulation tests. Among the advantages of such a test are that the patient does not receive ionizing radiation; it is independent of the presence of iodide-or mercury-containing compounds; and it minimizes the number of patient visits. In remote areas the patient need not attend the specialist centre, because the T.S.H. injection can be given and the serum samples taken locally, the samples then being sent to the appropriate laboratory for assay.