As shown in a preceding paper, only exocytotic vesicles conveying newly synthesized protein to the follicle lumen remain in the apical part of rat thyroid follicle cells following elimination of TSH secretion. In the present paper the effect of TSH on these exocytotic vesicles was investigated. TSH secretion was suppressed by administration of thyroxine for 2 days. In the electron microscope administration of TSH was seen to induce well-known signs of endocytosis, such as formation of pseudopods and colloid droplets. In addition, a previously unrecognized change was noted, namely a progressive decrease in the number of exocytotic vesicles. At 5 min after TSH the number was obviously reduced and at 20 min less than 10% of the original number of vesicles remained. Quantitative electron microscopic autoradiography after administration of [3H]leucine showed that TSH caused, concomitant with the disappearance of vesicles, a transfer of radioactivity from the apical region of the follicle cell to the periphery of the follicle lumen. The distribution of labeled protein in thyroid subcellular fractions was studied 1.5 h after administration of [14C]leucine. At 5 min after administration of TSH there was an increase of protein-bound label in the supernatant fraction, containing the luminal colloid, and a corresponding decrease of label in the particle fraction which contained most of the cell organelles, including the exocytotic vesicles. This TSH-induced redistribution of labeled proteins was more pronounced at 10 min and still more at 20 min and appeared to be dose-dependent. These observations taken together are considered to justify the conclusion that TSH induces transfer of newly synthesized protein from the follicle cells to the follicle lumen by exocytosis (i.e., emptying of specific apical vesicles). It is suggested that a causal and functional interrelation may exist between the exocytotic and endocytotic processes.