[en] To determine the site of iodination of thyroglobulin, electron microscopic autoradiography of ¹²⁵I was made using in vitro mouse thyroid gland. The effect of TSH and dbcAMP on fine structure was also examined in cultured mouse thyroid gland and compared with those on in vivo mouse thyroid. When cultured in the medium containing TSH, the thyroid gland showed numerous pseudopods on the cell surfaces of the follicular epithelial cells and increase of colloid droplets in the follicular epithelial cells. Similar changes were also noted in the mouse thyroid gland stimulated by TSH in vivo. dbcAMP also caused pseudopod formation and increase in intracellular colloid droplets in cultured mouse thyroid gland but did not cause these changes in mouse thyroid gland in vivo. In the thyroid gland incubated in the medium containing ¹²⁵I, silver grains appeared over the periphery of the follicular lumen at 30 seconds. With lapse of time, grains over the follicular lumen increased very rapidly until the lumen was filled with numerous grains at 60 minutes of incubation. Grains over the follicular epithelial cells were always scanty and showed no constant localization on the specific intracellular organelle. Since it has been known that the synthesis and release into the follicular lumen of the thyroglobulin is a process requiring at least 15 to 40 minutes, the appearance of silver grains over the follicular lumen at 30 seconds seemed to indicate the follicular lumen as the principal site of thyroglobulin iodination. When TSH was added to the culture medium containing ¹²⁵I for 60 minutes, grains were found over the follicular lumen and intracellular colloid droplets. (author)