Hypothalamic neuropeptide Y (NPY) has been implicated in the regulation of energy balance and reproduction, and chronically elevated NPY levels in the hypothalamus are associated with obesity and reduced reproductive function. However, it is not known which one of the five cloned Y receptors mediates these effects. Here we show that crossing the Y4 receptor knockout mouse (Y4(-/-)) onto the ob/ob background restores the reduced plasma testosterone levels of ob/ob mice as well as the reduced testis and seminal vesicle size and morphology to control values. Fertility in the sterile ob/ob mice was greatly improved by Y4 receptor deletion, with 100% of male and 50% of female Y4(-/-),ob/ob double knockout mice producing live offspring. Development of the mammary ducts and lobuloalveoli was significantly enhanced in pregnant Y4(-/-) and Y4(-/-),ob/ob females. Consistent with the improved fertility and enhanced mammary gland development, gonadotropin releasing hormone (GnRH) expression was significantly increased in Y4(-/-) and Y4(-/-),ob/ob animals. Y4(-/-) mice displayed lower body weight and reduced white adipose tissue mass accompanied by increased plasma levels of pancreatic polypeptide (PP). However, Y4 deficiency had no beneficial effects to reduce body weight or excessive adiposity of ob/ob mice. These data suggest that central Y4 receptor signaling specifically inhibits reproductive function under conditions of elevated central NPY-ergic tonus.