Twenty women with anorexia nervosa were investigated at varying stages during weight gain. Basal prolactin and TSH and prolactin responses to TRH were normal and unrelated to body weight. LH, FSH and 17 beta oestradiol were low in emaciated patients and rose with weight gain. There was no correlation between serum gonadotrophin and prolactin concentrations. T3 and T4 concentrations were low but T3 rose with weight gain during refeeding over 4-6 weeks, whereas T4 remained low. A positive correlation was found between the TSH response to TRH and body weight. The abnormalities in the hypothalamic-pituitary-thyroid axis were similar to those seen in a variety of chronic illnesses and appear to be unrelated to the amenorrhoea. The failure of restoration of normal function at least after short-term refeeding requires further investigation. It was concluded that the amenorrhoea in anorexia nervosa is not associated with changes in prolactin secretion but is determined primarily by changes in the hypothalamic-pituitary-gonadal axis. These changes are induced largely by nutritional factors but psychological factors may also be involved.