We studied plasma concentrations of thyrotropin (TSH), prolactin and growth hormone (GH) after injection of 500 microgram of thyrotropin-releasing hormone (TRH) in 10 patients with acute anorexia nervosa, subsequent to initial nutritional stabilization and again after weight recovery. Plasma thyroxine levels were normal throughout, whereas plasma triiodothyronine levels were low initially but rose with weight gain. The TSH secretory response to TRH was delayed and prolonged during the initial study but showed a normal overall quantitative response, except for two patients who showed no TSH rise. Following weight gain the TSH response was more rapid, and positive correlations were found between body weight and peak TSH levels and rapidity of TSH response. Six of 10 patients, however, continued to exhibit a delayed TSH peak response, the average response was markedly increased in comparison with that in normal females. The prolactin response curves were normal at both times. Rises in GH following TRH were observed in two patients prior to and in one patient after weight gain. We conclude that acute anorexia nervosa, with its concomitant profound weight loss, is accompanied by abnormalities in the hypothalamo-pituitary-thyroid axis, which are reversed only in part with improvement in the illness and weight gain, suggesting the persistence of disordered neuroendocrine function in this illness.