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[¹⁴C]Inulin, ³⁵SO₄^2- and ³⁶Cl^- spaces, ¹³¹I^- thyroid: serum (T:S) ratio, histological measurements of follicles and electrolyte content of the thyroid gland were determined in male rats 3–17 weeks of age. The thyroid [¹⁴C]inulin space of animals of different ages was not significantly different. The thyroid ³⁵SO₄^2- space, however, varied with age. The youngest rats had the largest ³⁵SO₄^2- space. Histological measurements indicated that in the thyroid follicles the epithelial cells were much thinner and the luminal volume was much larger in the younger rats than in older animals. In the older rats, the ³⁵SO₄^2- space was equal to the sum of [¹⁴C]inulin (stromal) and the histologically determined luminal spaces. In the younger animals, although the ³⁵SO₄^2- space was larger than in the older animals, it was still not large enough to equal the sum of the volumes determined by the [¹⁴C]inulin and the direct histological measurements of the lumen. This is due to the high colloid content in the follicular lumen of the young rats. The lower thyroid ¹³¹I^- T:S ratio in younger rats also indicated that the thyroid gland was less active than in older animals. The volume of distribution of thyroid Cl^- calculated from the chemically determined data of the plasma and of the thyroid tissue concentrations of rats of different ages was the same as that determined by ³⁶Cl^-.