Intravenous administration of thyrotrophin-releasing hormone (T.R.H.) induces a prompt rise in immunoassayable thyroid-stimulating hormone (T.S.H.) and triiodothyronine (T3) in normal man. Basal T.S.H. levels are high in primary hypothyroidism and rise dramatically after T.R.H. In patients with hypothyroidism secondary to pituitary disease basal T.S.H. levels are low and show no increase with T.R.H. In contrast, 2 patients with hypothalamic hypothyroidism had nil basal levels of T.S.H., which rose normally after T.R.H. administration. Basal T3 levels were low in all forms of hypothyroidism and did not rise after T.R.H. administration. The failure of T3 to increase normally after T.R.H. in the 2 patients with hypothalamic hypothyroidism in the face of a normal T.S.H. stimulation test suggests a diminished thyroidal reserve in these patients. Intravenous T.R.H. administration may prove of value in the simultaneous assessment of pituitary and thyroidal reserve.