Physiological brain aging is characterized by important biochemical and structural changes and by the unbalance among the different neurotransmitters and neuromodulators. The study of the circadian organization of neuroendocrine functions may be considered a clinically reliable tool to investigate the changes of the CNS and particularly of the limbic-hypothalamic system occurring in aged people. The circadian rhythms of plasma melatonin, ACTH and cortisol and of oral temperature were studied in 16 clinically healthy women aged 66-90 years and in 14 young controls aged 20-30. In addition, the effect of dexamethasone on the plasma cortisol circadian rhythm and the cortisol response to Synacthen pulse intravenous injection were evaluated. All subjects were studied as inpatients, with the same synchronization to the hospital life schedule. When compared with young controls, elderly subjects exhibited a reduction of the mean level and of the amplitude of the circadian rhythm of oral temperature, an increase of the mean level of ACTH and cortisol rhythms and a selective impairment of melatonin nocturnal secretion. Furthermore, elderly subjects showed a reduced sensitivity to the dexamethasone suppression test, by comparison to young controls. These changes were age-related and they may depend either on CNS modification or on alterations of the hormonal metabolic clearance.