ABSTRACT The uptake by thyroid tissue of radioactively labelled macromolecules was studied in vitro to elucidate the mechanism of endocytosis of thyroglobulin by the follicular cell. Minced sheep thyroid was incubated with [125I]polyvinylpyrrolidone (PVP*), [3H]inulin, [125I]thyroglobulin or [125I]fibrinogen. Tissue radioactivity comprised a "weakly bound" component, removed by prolonged washing and a residual "firmly bound" component. TSH, 5 mU/ml, increased the magnitude of both these components. The TSH effect on PVP* uptake demonstrated hormone and tissue specificity and was reproduced by dbcAMP. The quantity of PVP* firmly bound rose with time of incubation with a rapid initial uptake followed by a slower "linear" increase. Vinblastine markedly reduced the "linear" rate of uptake of firmly bound radioactivity. The release of non-protein bound 125I into the medium during incubation with [125I]fibrinogen was stimulated by TSH. The rapid initial uptake of firmly bound radioactivity probably represents firm adsorption, the subsequent linear rate endocytosis.