A review of the literature (194 papers) on the influence of thyroid treatment of various organisms reveals several interesting facts. Thyroid feeding of the vertebrates shows that the thyroid which normally is an organ of internal secretion, delivering its product to the body parenterally, may, if eaten, induce definite and considerable changes in the organism both immediately and over considerable periods of time. These changes, which are seen as accelerations of metamorphosis in amphibians, or of either the moulting cycle or feather production in birds, or of growth or of milk or milk fat secretion in mammals, are all manifestations or expressions of the metabolic regulator function of the gland which is its basic job. Thyroid or the isolated hormone, thyroxin, in association with isolated vertebrate, invertebrate or plant tissues, causes an acceleration in anabolic and katabolic processes of metabolism. In this respect, the 3 types of tissues may be considered near enough alike physiologically to respond to the same endocrine substances. Hence, thyroid undoubtedly exerts its metabolic regulator activity on all protoplasm irrespective of the organism in which it occurs. Invertebrates (protozoa and insects at least) although they possess no thyroid structure, will, in some cases, respond to the action of the vertebrate hormone. When there is a response, the reaction is quite similar to that seen in the vertebrates[long dash]an increased rate of metabolic processes. The abundance of negative evidence in the literature on thyroid feeding of invertebrates, however, prevents the drawing of any specific generalizations on the subject; hence there is a need for carefully controlled experimentation in this field.