Alterations in periodical functions are known to occur in aging and may be regarded as markers of the aging process itself. Melatonin and Thyroid Stimulating Hormone (TSH) circadian periodicities were studied in 22 aged subjects and in 13 adult controls. The study of rhythmicity was performed by the Cosinor analysis. Elderly subjects were hospitalized because of various concomitant diseases. Circadian periodicity of both hormones was disrupted in the aged group, and the deterioration of melatonin periodicity was significantly correlated with the decay in cognitive functions, quantified by the Mini Mental State evaluation. Diabetes was also found to affect, though not significantly, melatonin, but not TSH, periodicity. Melatonin and TSH nocturnal peaks were decreased in aged people. TSH oscillation amplitudes were inversely correlated with age. No correlation was found between melatonin and TSH secretory features both in adult and in aged subjects.