The purpose of the present study was to assess thyroid-stimulating hormone (TSH), prolactin, and growth hormone responses to TRH stimulation in 12 congenitally hypothyroid children adequately treated with L-thyroxine from the first weeks of life. Although clinically euthyroid, six of these children were found to have abnormally high basal serum TSH concentrations despite clinical euthyroidism. Serum triiodothyroxine and L-thyroxine concentrations were normal and did not differ whether the children had elevated or normal basal serum TSH. All six of the children with high basal TSH had an exaggerated TSH response to TRH and 4 of them also had an augmented prolactin response to TRH. The children with normal basal TSH concentrations had normal TSH and prolactin responses to TRH. An abnormal ("paradoxical") elevation of growth hormone concentration in response to TRH was found in four of seven children in a separate group of patients who had prolonged, untreated primary hypothyroidism, but such responses were not found in any of the adequately treated children. These findings suggest the following conclusions: 1) the phenomenon of high serum concentrations of TSH in conjunction with normal L-thyroxine and triiodothyronine levels (and clinical euthyroidism), is prevalent in congenital hypothyroid patients. 2) These patients have an exaggerated response of their pituitary thyrotroph and lactotroph cells to TRH, presumably caused by selective and relative resistance of these cells to the inhibitory effects of thyroid hormones. 3) Congenital hypothyroidism is not associated with abnormal somatotroph cell responses to TRH.