The thyrotrophin (TSH) response to thyrotrophin-releasing hormone (TRH 200 microgram intravenously was studied in 19 patients with unipolar depression, 12 with bipolar depression, 14 with mania, and 5 with mixed manic-depressive illness. The TSH responses were decreased in all of these affective disorders compared to those found in 10 patients with reactive depression, 5 with reactive paranoid psychosis, 14 with neurotic depression, and 60 controls. The decrease of the TSH response in manics could not be accounted for by the effects of neuroleptic drugs. The TSH response in the groups with reactive depression, reactive paranoid psychosis, and neurotic depression, respectively, did not differ significantly from that found in controls. With the exception of a decrease in serum T3 level and free T3 index in the manics, no significant differences in serum T3 level or in free T3 and T4 indexes were found between the groups. Changes found in serum T4 level were due to changes in the thyroxine-binding proteins.