Most women experience time-limited and specific mood changes in the days after birth known as the maternity blues (Blues). The maternal hypothalamic-pituitary-adrenal (HPA) axis undergoes gradual changes during pregnancy because of an increasing production of placental corticotrophin-releasing hormone (CRH). The abrupt withdrawal of placental CRH at birth results in a re-equilibration of the maternal HPA axis in the days post-delivery. These changes may be involved in the aetiology of the Blues given the central role of the HPA axis in the aetiology of mood disorders in general, and in perinatal depression in particular. We aimed to test the novel hypothesis that the experience of the Blues may be related to increased secretion of hypothalamic adrenocorticotrophic hormone (ACTH) secretagogue peptides, after the reduction in negative-feedback inhibition on the maternal hypothalamus caused by withdrawal of placental CRH. We therefore examined hormonal changes in the HPA axis in the days after delivery in relation to daily mood changes: our specific prediction was that mood changes would parallel ACTH levels, reflecting increased hypothalamic peptide secretion. Blood concentrations of CRH, ACTH, cortisol, progesterone and oestriol were measured in 70 healthy women during the third trimester of pregnancy, and on days 1-6 post-delivery. Blues scores were evaluated during the postpartum days. Oestriol, progesterone and CRH levels fell rapidly from pregnancy up to day 6, whereas cortisol levels fell modestly. ACTH concentrations declined from pregnancy to day 3 post-delivery and thereafter increased up to day 6. Blues scores increased, peaking on day 5, and were positively correlated with ACTH; and negatively correlated with oestriol levels during the postpartum days, and with the reduction in CRH concentrations from pregnancy. These findings give indirect support to the hypothesis that the 'reactivation' of hypothalamic ACTH secretagogue peptides may be involved in the aetiology of the Blues.