Aging in men is associated with a decline in trophic factors such as testosterone (T), alterations in body composition and impaired energy and body weight regulation. We performed studies to investigate the mechanisms underlying age-related changes in the neuroendocrine control of testis function, body composition, food intake and body weight in the Brown Norway (BN) rat. We found that similar to aging men, male BN rats demonstrate both primary and secondary testicular failure with aging without confounding age-related tumors, hormonal changes and systemic illnesses. With aging, these animals have blunted circadian variations in luteinizing hormone (LH) and T, and decreased hypothalamic gonadotropin-releasing hormone (GnRH) synthetic capacity with preserved pituitary gonadotropin responses to GnRH. We found that aging male BN rats have increased peripheral and visceral adiposity associated with increased insulin and leptin levels, and decreased relative lean body mass and muscle mass. We found that these rats exhibit reduced food intake and body weight gain associated with decreased hypothalamic neuropeptide Y (NPY) gene expression in the arcuate nucleus (ARC), both during ad-libitum feeding and after a 72-h fast. Recently, we found that old male BN rats treated chronically with troglitazone, an insulin sensitizer, lowered high insulin and leptin levels, decreased body fat, and corrected the blunted food intake and body weight gain response to fasting without affecting basal ARC NPY gene expression. These findings suggested that hyperinsulinemia and/or hyperleptinemia associated with aging may contribute to the age-related impairment in energy and weight regulation. Our studies suggest that the aging male BN rat is an excellent model to investigate the mechanisms underlying the age-associated changes in the neuroendocrine control of body composition, energy intake and body weight.