The effects upon basal hormone levels and neuroendocrine responses of a weight reducing diet allowing 1200 kcal daily were determined in male and female volunteers. Thyrotropin (TSH) responses to thyrotropin-releasing hormone (TRH) were unchanged in men but attenuated in women; this effect was associated with a fall in basal TSH in women, not in men. Rates of non-suppression of cortisol in response to oral dexamethasone (1 mg) were unchanged during dieting although basal morning cortisol levels rose in males and females. The implications for the use of the TRH test and the dexamethasone suppression test (DST) in depressive illness are discussed.