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Among the several mechanisms thought to be active in the function of the thyroid gland is one which controls the viscosity of the colloid of the follicles. It is obvious that if the colloid were highly viscous, reabsorption of active substance from it would be delayed, whereas if it were less viscous, diffusion of active substance would be accelerated. De Robertis [1941] showed that the viscous colloid of normal guinea-pigs and rats becomes more fluid when the thyroid gland is activated, and less fluid when the activity of the organ is depressed. This was explained by the occurrence in the colloid of 'cathepsin', a protein-digesting activity, which varies directly with the fluidity of the colloid. These findings suggest that the changes in the viscosity of the colloid may be attributed largely to variations in the concentration of the colloid protein brought about by proteolytic hydrolysis (and absorption of the products)