The TSH response to TRH (delta max TSH) and the serum concentrations of free thyroxine (FT4), 3,5,3'-, and 3,3',5'-triiodothyronine (FT3 and FrT3) were studied in two groups of patients with endogenous depression before and after clinical recovery following electroconvulsive treatment (ECT). Before ECT, the patients from group 1 (n = 17) had a reduced delta max TSH (p less than 0.01), which after ECT rose to values not different from those found in controls. FT4 levels were elevated before ECT (p less than 0.01), and they decreased after ECT (p less than 0.05) to levels similar to those found in controls. FT3 and FrT3 levels were not different from the control values, but FrT3 decreased during ECT (p less than 0.01). In group 2 (n = 19), delta max TSH was reduced both before (p less than 0.02) and after (p less than 0.01) ECT. FT4 levels were increased both before and after ECT (p less than 0.02). Both parameters were unaffected by ECT. The data are compatible with the assumption that the decreased TSH response to TRH found in patients with endogenous depression is secondary to an increase in circulating FT4.