One hundred ninety four children, 11 +/- 1.5 years of age and 166 elderly men and women, 77 +/- 8 years of age were studied over one or (in the case of some of the elderly subjects) over several (up to 4) 24-hours spans. All subjects were diurnally active and rested at night and followed their regular three meal pattern. The subjects were studied in subgroups of 20-25 during all four seasons of the year. During each study, blood was collected at 4 hour intervals over one 24-hour span (6 samples). Circadian and circannual variations were found and described by cosinor analysis in the children as well as in the elderly subjects. The children with endemic goiter (134) as compared to those without endemic goiter (60) showed a slight circadian phase advance in plasma total and free T3, a lower circadian amplitude of total T4 concentrations and the absence of a detectable circadian rhythm in free T4. The children with goiter showed a phase delay in serum TBG. There was no difference between the children with and without goiter in the circadian MESOR of any thyroid parameter or of TSH. The children with endemic goiter in the region of Dimboviţa, Romania, are in clinical and biochemically euthyroid condition with some slight poral abnormalities of thyroid function. Seasonal variations in children and elderly patients showed the highest values of TSH during summer and fall, while the highest values in the plasma concentrations of thyroid hormones were found during the cold season of the years. Thyroglobulin in the children showed a circadian rhythm but no seasonal variation.