The skin of rats after adm. of thyroid gland or thyroxine over a considerable period shows the following changes in its chemical composition; its cholesterol, chloride and water contents rise, while the neutral fat content diminishes. The skin of rats in exptl. hyperthyroidism has an increased water-binding power. By facilitating the loss of heat from the surface of the skin these chemical changes are related to heat regulation. The low fat content enhances the heat conduction and the high cholesterol content (up to 300% of the normal values) favors the water inhibition of the skin and the succeeding evaporation of water. Possibly also loss of I through the skin is favoured. No definite changes occur in the cholesterol content of the lungs and the blood during thyroid adm.