The ability of thyroid tissue to take up labeled thyroglobulin (Tg*), as a first step leading to the release of hormones, has been studied in vitro. Slices of pig thyroid incubated in the presence of 125I-Tg were able to bind a large quantity of this protein. After incubation, part of the accumulated radioactivity was released in the presence of more concentrated unlabeled Tg (2.5% w/v). The amount of the nondischargeable Tg* increased with time of incubation, Tg concentration and temperature (energy of activation 11,800 cal/mole). At low Tg concentrations a saturation of membrane sites by the protein was observed (adsorptive endocytosis, Km=2.9×10-6M), while at higher concentrations only engulfment of the soluble material occurred (liquid pinocytosis). The endocytic process is not specific for Tg: albumin is taken up in the same way, but the affinity of the cells for this protein is lower than for Tg. Large amounts of albumin in the medium lead to a competitive inhibition of the Tg uptake. One third of the dischargeable radioactivity was soluble in trichloroacetic acid. This release of soluble material increased with the temperature, the Tg concentration and the duration of the previous incubation. Chromatographic studies on the discharge medium after different incubation time revealed the presence of increasing amount of iodothyronines (13.9% of the released radioactivity after 2 hr of incubation). These results are consistent with the view that, in vitro, the follicular cells exposed to Tg* are able to bind and then to engulf this soluble substrate. The protein is thereafter metabolically processed in the cellular structures (colloid droplets, phagolysosomes). Hormonal products and other iodocompounds are finally released into the medium. (Endocrinology88: 389, 1971)