Follicles from rat and pig thyroid glands were isolated by collagenase digestion. The epithelial cells of isolated follicles maintain their structural and functional polarity as shown by incorporation of 3H-Leu and autoradiography. To trace the fate of surface membrane, isolated follicles were opened, stimulated with thyrotropin and incubated for various time intervals with cationized ferritin (CF), uncharged dextran, native ferritin (NF), and latex spheres (0.5 .mu.m in diameter) which were either pre-coated with CF or added together with CF. Uncharged dextran and native ferritin did not bind to the luminal cell membrane, were taken up in small amounts and accumulated in lysosomes; anionic NF was not found in Golgi cisternae in contrast to uncharged dextran which occasionally reached a few Golgi stacks. CF bound rapidly and in clusters to the luminal plasmalemma, preferentially to coated pits, was taken up by endocytosis, accumulated in lysosomes after 5 min and reached the Golgi cisternae after 30 min. Latex spheres were taken up by engulfment through fusion of microvilli and reached the lysosomes. CF particles coating the latex spheres may detach at this station and reach the Golgi cisternae. The route of small tracers depends on the charge of the tracer, in agreement with results obtained by Farquhar. Vesicles carrying NF can be traced to lysosomes only, whereas vesicles containing uncharged dextran or, more conspicuously, CF also fuse with Golgi membranes. Large tracers (latex beads) reach only the lysosomes, but CF taken up with them may move to Golgi cisternae.