The discovery of leptin has generated an extraordinary interest in the field of obesity but also in the understanding of the relationship between metabolic status and the neuroendocrine system. Following the initial demonstration that leptin administration to fasting mice can 'protect' neuroendocrine secretions and prevent the changes that are associated with fasting, the concept has emerged that a normal leptin secretion is a prerequisite for normal neuroendocrine secretions. Several unfavorable metabolic situations are associated with low plasma leptin, increased secretion of hypothalmic neuropeptide Y (NPY), and hypogonadism, and a causal relationship has been evoked. Severe dietary restriction in juvenile female rats is associated with low plasma leptin and sexual immaturity. Cessation of food restriction leads to immediate increase in plasma leptin followed 4 days later by vaginal opening. If food restriction is maintained, central leptin infusion can induce sexual maturation, thus demonstrating that leptin can act as a signal for the onset of puberty. In untreated type-I diabetic rats, hypogonadism is associated with very low plasma leptin and increased hypothalmic NYP synthesis and oestrous cyclicity. Fasting rapidly inhibits growth hormone (GH) secretion in association with low plasma leptin and elevated hypothalmic NPY. Central infusion of leptin to fasting rats was able to completely prevent the collapse of GH secretion and to maintain a normal low NPY synthesis. In summary, normally elevated plasma levels appear to be a prerequisite for normal GH and gonadotropin secretion in the rat. Degradation of metabolic conditions results in a rapid reduction of circulating leptin that could represent the signal for several alterations of neuroendocrine secretions. At the level of the hypothalamus, leptin could act on NPY neurons to transduce part or all of this 'metabolic' message. The possibility that changing plasma levels for leptin also affect peripheral endocrine targets, such as pituitary, ovary, adrenal or pancreas, is likely since these endocrine organs express functional long-term leptin receptors.