Plasma HTSH was evaluated by radioimmunoassay after i. v. injection of 600 ug of TRF in 14 patients with pituitary tumors, in 6 normal volunteers and in 2 subjects with mild primary myxedema. 4 patients ( 3 acromegalics and 1 chromophobe adenoma)had secondary hypothyroidism: 2 of them showed a normal response, 1 had a prolonged and exaggerated increase in plasma HTSH, 1 had a blunted response. This patient,re-examined after surgery, showed an exaggerated response. 10 patients (5 acromegalics, 4 chromophobe adenomas, 1 craniopharyngioma) were euthyroid : plasma HTSH did not increase after TRF in 1 case, rosenormally in 5, while in 4 cases had a prolonged or exaggerated response. On the whole,6 patients showed this characteristic pattern of response, quite similar to the one observed in mild primary myxedema. In 2 of these patients HTSH -125I half-life and plasma clearance resulted within the normal range. These data suggest a hypothalamic rather than pituitary impairment of TSH secretion.