1. During the 1st and 2nd day of cultivation of the whole thyroid gland in the Lindbergh apparatus, perfused by a medium containing 20 or 40 per cent homologous serum diluted with glucose-Tyrode, the state of the thyroid epithelium remains histologically normal and even the most delicate cytological details retain their normal aspect.

2. Cultivation over a period up to 6 days need not cause any serious histological alteration of the gland; but cytological examination reveals an increasingly damaged condition of the mitochondria and the Golgi apparatus. These changes seem not to interfere with the life of the cells, however; epithelial culture from the perfused organs yields positive results.

3. The process of cultivation is liable to cause a slight morphological reaction in the thyroid epithelium, which resembles a feeble stimulation of the tissue.

4. Addition of the thyro-stimulating factor from the anterior pituitary to the perfusion fluid causes a stimulation of the cultivated thyroid epithelium which is comparable to, but not so marked as, the corresponding reaction in the living animal.

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