Adm. of graded doses of a thyrotrophic extract to frog larvae in several intraperit. injs., totalling 0.000078 to 0.05 cc. of the hormone, can induce metamorphosis in the stasis (starved, non-metamorphosing) tadpole. This is characterized by progressive body-weight loss, increase in hind limb length, and by activation of the thyroid. Histological examination of the thyroid in controls invariably shows a resting gland as compared with hyper-plastic changes in hormone treated animals. The capacity for activation appears to be limited to material of pituitary origin. No thyroid stimulation was produced with glucose, glucosamine, gluthathione, cysteine, wheat germ oil, inorganic phosphate, or thyroxine. Total starvation (16-18 days) prior to treatment does not alter appreciably the degree of thyroid activation nor the rate of accelerated metamorphosis. A tadpole unit of thyrotrophic activity is tentatively defined. Direct comparison with the immature guinea pig indicates that the tadpole responds to approx. 1/60 to 1/120 the amt. of thyrotrophin which can be detected histologically in the guinea pig. If body wt. is taken into consideration, the sensitivity is of the same order of magnitude.