The antithyrotropic activity of freeze-dried-extracts from Lithospermum officinale (Lith. off. FDE) was investigated in the rat. When administered together with TSH, Lith. off. FDE blocked the TSH-induced increase in endocytotic activity of the thyroid glands followed by a strong decline of thyroid hormone levels. Furthermore, when Lith. off. FDE was injected alone it caused a decline in endogenous TSH-levels as well as in thyroidal secretion and thyroid hormone levels. The efficacy of the extract in blocking thyroid secretion was compared to that of potassium iodide and it was found that the effect of Lith. off. FDE was of more rapid onset and of longer duration, suggesting that the FDE may have a different mode of action from that of KJ. A specific interaction between TSH and the active constituents of the plant extract is discussed. Experiments on thyroidectomized and T4 substituted rats have demonstrated as an additional pharmacodynamic effect of Lith. off. FDE an inhibition of peripheral T4-deiodination.