Thyroids from rat fetuses of different ages (from day 14 to day 19 of gestation) were transplanted to organ culture for 2 days, with or without added thyroid-stimulating hormone (TSH) in the medium. Thyroid tissue from 14-day fetuses that initially consisted of irregularly arranged cell cords did not form follicles when cultured in the presence or absence of TSH. Thyroids from 15- and 16-day fetuses initially consisted of epithelial cell masses. When cultured in the presence or absence of TSH they formed follicles, and a majority stored small amounts of colloid. In thyroid transplants from 17-day fetuses, the response to TSH appeared as a significant increase in the follicular diameter and cell height. Thereafter, in all transplants cultured in the presence of TSH, both the follicular diameter and the cell height were markedly greater than in the transplants cultured in the absence of added TSH. These results suggest that the initial formation of thyroid follicles is independent of TSH and that, once developed, follicles become able to respond to TSH.