The study of the neuroendocrine changes occurring in aging may give information about the CNS functions, and also explain the impaired plasticity of the aged organism. In 16 elderly women and in 14 young controls, the circadian rhythms of plasma melatonin, GH, PRL, ACTH and cortisol, and of oral temperature were simultaneously studied. The plasma cortisol circadian rhythm was also evaluated after DXM administration (1 mg orally at 23:00). The circadian profile of all the bioperiodic functions evaluated was clearly flattened in elderly subjects, and an impairment of the hormonal nocturnal secretion of GH, PRL and melatonin was apparent in elderly subjects when compared to young controls. The plasma ACTH levels throughout the 24-hour cycle were significantly higher in elderly than in young subjects. The cortisol circadian profile exhibited significantly higher values in the evening- and night-time in elderly subjects, compared to young controls; the cortisol nadir values were significantly age-related. A reduction of the sensitivity to DXM inhibition occurred in the elderly group. Both the selective impairment of nocturnal melatonin secretion, and the reduction of hypothalamo-pituitary-adrenal (HPA) sensitivity to steroid feed-back might be considered as markers of aging brain. The neuroendocrine alterations of physiological aging may be ascribable to both the structural and neurochemical changes occurring in the CNS.