Thirteen circadian rhythms in plasma hormone levels (ACTH, aldosterone, cortisol, C-peptide, insulin, DHEA-S, estradiol, LH, prolactin, 17-OH progesterone, testosterone, T4 and TSH) were studied in April 1981 in 25 males and 25 females 57 to 91 years of age, institutionalized at the Berceni Hospital for the aged. The radioimmunoassays and the statistical rhythmometric evaluation by the cosinor procedure were done at S. Paul-Ramsey Medical Center, St. Paul, MN, USA. Circadian rhythms were found and quantified for each of these variables. Elderly subjects of both sexes thus maintain a circadian time structure of their endocrine system as a group phenomenon. In comparison with previous data from the Endocrine Rhythms Laboratory ("C. I. Parhon" Institute) and series of younger subjects studied in Minnesota (St. Paul-Ramsey Medical Center) as well as in comparison with data published from other centers, the aged seem to experience an earlier arousal of their endocrine system which may be related to certain disturbances of old age e.g. of sleep. The latter observations will have to be confirmed by additional studies which take both circadian and circannual variations in account.