Physiological challenges facing the perinatal individual are huge. This is partly because of the marked and abrupt change from the intrauterine to the extrauterine environment, but also because the physiological responses to that change depend upon systems whose maturation is affected markedly by birth. Perinatal maturation appears, therefore, both to precede and accompany the physiological adaptations required for neonatal survival. This review does not provide a comprehensive coverage of perinatal physiology but directs attention towards evaluating constraints on foetal and neonatal thermogenesis, especially those imposed by post-natal availability of metabolic fuels in body reserves deposited before birth and acquired in colostrum and milk after birth. The relative importance of the supply of carbohydrate from body reserves and colostrum is examined. Carbohydrate is the major rate-limiting substrate with respect to heat production. It is shown that in the normal birth environments of several species, including the horse, colostral supply of lactose is not adequate to meet the entire carbohydrate needs of the neonate. It must, therefore, draw on its body glycogen reserves to sustain the required rates of heat production during the first day after birth. Gastrointestinal maturation and growth play a major part in the initial and continued provision of substrates for general metabolism and growth. Pre-natal and post-natal development of the stomach and intestines in a number of species are considered, including their disproportionately large growth compared with the body as a whole during the first 1-3 days after birth. Possible contributions to gastro intestinal growth and body growth of milk-borne growth factors such as epidermal growth factors and the insulin-like growth factors are considered. The role, as a possible significant stimulus to gastrointestinal growth and maturation immediately after birth, of the high levels of these growth factors in colostrum is evaluated.