One-hundred and ninety-four children (11 +/- 1.5 years of age), 43 young adult subjects (21 +/- 2 years of age) and 149 elderly subjects studied in 278-24 hour profiles (77 +/- 8 years of age) were studied over one or several 24-hour spans. All subjects followed a diurnal activity pattern with rest during the night. Blood and urine were collected at 4-hour intervals over a 24-hour span (6 samples). The circadian rhythms of 22 endocrine parameters were explored in plasma and five in urine. In the children and the elderly subjects, the investigations extended over all four seasons allowing also the study of seasonal variations or circannual rhythms. The circadian rhythms were characterized by population mean cosinor for each group of subjects. The circannual rhythms were explored by single cosinor, by one and two way analysis of variance applied to the circadian means obtained during each season and by comparison of the circadian rhythm parameters obtained during the four seasons by the parameter test described by Bingham et al. Circadian rhythms were found in most and circannual rhythms in many parameters. The rhythm characteristics are presented as cosinor summaries and acrophase charts. Although there were certain differences in some rhythm parameters between the age groups, the elderly showed a remarkable maintenance of their circadian time structure. In the circannual frequency range, the elderly men showed more circannual periodicity as group phenomenon than the women. Many endocrine rhythms show high amplitude rhythms which have to be taken into account in the selection of the time for endocrine testing and in the interpretation of the results.