A review of some hormonal and metabolic changes occurring during the four stages of the perinatal period is presented. Glucocorticoids and insulin are the hormones that mediate liver glycogen accumulation during late fetal stage. In the presuckling period, muscle glycogenolysis supplies the lactate moieties that are oxidized by the neonatal tissues, representing the alternative substrate until glucose and ketone bodies become available. The postnatal increase in plasma catecholamine concentrations and the decrease in the insulin/glucagon ratio triggers liver glycogenolysis and gluconeogenesis, and hence postnatal hypoglycemia is reversed. In the suckling period, the oxidation of fatty acids, ketone bodies utilization and active gluconeogenesis supply the bulk of the energy and carbon components required to support the rapid growth rate of this period. The increase in the insulin/glucagon ratio that occurs with the change to a carbohydrate-rich diet starts the induction of lipogenesis at weaning.