Hormones play a vital role in promoting growth and in re-ordering metabolic priorities among tissues in a wide variety of physiological situations important to animal production. During pregnancy and lactation, however, the hormonal requirements for growth promotion in the developing infant seem diametrically opposed to those necessary within the mother for support of pregnancy and later for the provision of milk to the suckling young. Fetal endocrine autonomy clearly plays an essential role in protecting prenatal development. Despite this, fetal metabolism cannot be isolated altogether from that of the mother because of the need for maintained nutrient transfer to the conceptus. Indeed, hormones secreted by the fetal placenta into the maternal circulation appear to play important roles in manipulating maternal metabolism to favour transfer of metabolic substrates such as glucose to the conceptus. Similarly, alterations in the secretion of growth hormone during lactation have been considered to play an important homeorhetic role favouring transfer of substrate to the mammary gland for milk synthesis. However, many aspects of these adaptive changes remain uncertain.