Leptin is a circulating protein produced by adipocytes that has been implicated in control of body weight through appetite regulation and control of reproduction, most likely through an effect on the central nervous system. From studies in mice, it is clear that the genetic background of the animal on which the mutation in the leptin gene is placed can influence how that mutation is expressed. Although the effects of leptin have been more thoroughly documented in nonprimate species than in primates, a few human families with genetic mutations of the gene for leptin or the leptin receptor show obesity and impaired fertility. There is conflicting evidence regarding the effects of leptin in male primates, but it appears that the metabolic and reproductive effects of low leptin levels caused by reduced energy intake in adult animals can be more readily alleviated by administration of energy substrates than by administration of leptin.