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Studies of the influence of age and sex on the concentrations of total thyroxine (T₄) and 3,5,3'-tri-iodothyronine (T₃) in serum and on the free T₄ and free T₃ indices, were conducted in Sprague–Dawley rats of the CD strain varying in age between 10 days and 12 months. Both sex- and age-related differences were found. In all age-groups studied, serum T₄ concentrations were higher in the male than in the female, whereas serum T₃ concentrations were higher in the female. In both sexes, concentrations of T₄ and T₃ in serum reached a peak early in life, between the first and second month of age, and declined thereafter. In addition, in both sexes the intensity of thyroid hormone binding, as judged from values of the in-vitro uptake of T₃, did not change with age, suggesting that free T₄ and T₃ concentrations in the serum display the same sex differences and age-related changes as do the concentrations of total T₄ and T₃. It remains to be determined whether these sex-and age-related alterations in serum thyroid hormone concentration are expressed in differences in the activity of various thyroid hormone-dependent processes.