Mild alterations in serum magnesium are said to be symptomless (D. N. S. Kerr in Cecil and Loeb, 1971). Cade (1964) reported high levels in depression but Frizelet al.(1969) found low values restored to normal with clinical recovery. Raised serum magnesium concentrations have been reported in manic patients (Nielson, 1964; Goodwin, Murphy and Bunney, 1968) and schizophrenics (Cade, 1964). These results suggested that serum magnesium had predictive value in the treatment of psychiatric patients, so we estimated pre- and post-treatment serum magnesium concentrations in a number of patients and investigated their relationships with diagnosis and clinical course.