The clinical utility of synthetic thyrotrophin-releasing hormone (TRH) in assessing thyrotrophin (TSH) and prolactin secretion was evaluated in 100 patients with pituitary and hypothalamic diseases. Age- and sex-specific criteria for normal TSH and prolactin responses were used. Thyrotrophin reserve was impaired in 56 of the patients, but in 23 of these the abnormality could not have been predicted, because the serum thyroid hormone levels were normal. Prolactin secretion was abnormal, either an elevated basal level or a subnormal response to TRH, in 62 of the patients; only in the 2 patients with galactorrhea could the abnormality have been predicted clinically. In certain situations hormonal abnormalities due to pituitary involvement could be distinguished from hormonal abnormalities due to hypothalamic involvement by the type of TSH or prolactin response. Measurement of the TSH and prolactin responses to TRH, it is concluded, can show many abnormalities of pituitary and hypothalamic function not otherwise detectable.