A B S T R A C T Synthetic thyrotropin-releasing hormone (TRH) was administered intravenously in a dose of 7Ag/kg to 20 normal children ages 4-13 yr. Serum thyroid-stimulating hormone (TSH) was measured by radioimmunoassay and rose from a mean value of 1.7 AU/ml (range = < 1.25-7.2) to a mean peak value of 21.5 ,uU/ml (5.2-33.2) at 15 or 30 min after administration.13 patients with idiopathic hypopituitarism and apparent normal thyroid function, ages 3-19 yr, responded to TRH in a manner very similar to the control subjects: TSH rose from a mean value of 1.8 /AU/ml (range < 1.25-4.3) to a mean peak value of 18.5 AU/ml (range = 9.5-45.0) which occurred between 15 and 60 min after TRH.13 idiopathic hypopituitary patients with documented thyroid deficiency were tested after thyroid therapy had been discontinued for a minimum of 10 days. The serum TSH values in 10 of 13 patients rose from a mean base line level of 2.2 AU/ml ( < 1.25-5.3) to a peak mean value of 32.5 itU/ml (9.6-61.3) between 30 and 120 min after TRH. In three patients, however, little or no TSH response was detected, even when serum thyroxine levels were extremely low. Similar to the latter group, three of five patients with hypopituitarism