The thyrotrophin (TSH) response to thyrotrophin-releasing hormone (TRH) 300 micrograms intravenously was studied in 25 patients with schizophrenia, 14 with depression, and 8 with mania. The rate of blunted TRH test results was zero in healthy controls, 64.3% in depression, 62.5% in mania, and 28.0% in schizophrenia. These differences of the rates between depression or mania patients and controls were significant, whereas the difference of rates between schizophrenias and healthy controls was not significant by X2 test. These results seemed to support the view that TRH stimulation test can be used as a biological marker to distinguish affective disorder and that both depression and mania is correlated with the decreased function of serotonin.