The serum triiodothyronine concentration was evaluated before and after thyrotrophin releasing hormone in fifty-six patients with hypothalamic-pituitary disorders (thirty-four had secondary hypothyroidism, twenty-two were euthyroid) and in twenty-four normal controls. Basal serum T3 was low in fifteen hypothyroid subjects and normal in the remainders. After TRH, serum T3 did not increase normally in twenty-five hypothyroid and in ten euthyroid patients; even the patients with normal or supranormal plasma TSH increase had significantly lower T3 responses than normal controls (P less than 0-0001 for hypothyroid, P less than 0-01 for euthyroid subjects). The finding of low T3 response to TRH in some euthyroid patients with hypothalamic-pituitary disorders can perhaps identify cases of preclinical secondary hypothyroidism, probably due to low biological activity of released TSH.