1. The rate of inactivation of crystalline trypsin solutions and the nature of the products formed during the inactivation at various pH at temperatures below 37 degrees C. have been studied. 2. The inactivation may be reversible or irreversible. Reversible inactivation is accompanied by the formation of reversibly denatured protein. This denatured protein exists in equilibrium with the native active protein and the equilibrium is shifted towards the denatured form by raising the temperature or by increasing the alkalinity. The decrease in the fraction of active enzyme present (due to the formation of this reversibly denatured protein) as the pH is increased from 8.0 to 12.0 accounts for the decrease in the rate of digestion of proteins by trypsin in this range of pH. 3. The loss of activity at high temperatures or in alkaline solutions, just described, is very rapid and is completely reversible for a short time only. If the solutions are allowed to stand the loss in activity becomes gradually irreversible and is accompanied by the appearance of various reaction products the nature of which depends upon the temperature and pH of the solution. 4. On the acid side of pH 2.0 the trypsin protein is changed to an inactive form which is irreversibly denatured by heat. The course of the reaction in this range is monomolecular and its velocity increases as the acidity increases. 5. From pH 2.0 to 9.0 trypsin protein is slowly hydrolyzed. The course of the inactivation in this range of pH is bimolecular and its velocity increases as the alkalinity increases to pH 10.0 and then decreases. As a result of these two reactions there is a point of maximum stability at about pH 2.3. 6. On the alkaline side of pH 13.0 the reaction is similar to that in strong acid solution and consists in the formation of inactive protein. The course of the reaction is monomolecular and the velocity increases with increasing alkalinity. From pH 9.0 to 12.0 some hydrolysis takes place and some inactive protein is formed and the course of the reaction is represented by the sum of a bi- and monomolecular reaction. The rate of hydrolysis decreases as the solution becomes more alkaline than pH 10.0 while the rate of formation of inactive protein increases so that there is a second point at about pH 13.0 at which the rate of inactivation is a minimum. In general the decrease in activity under all these conditions is proportional to the decrease in the concentration of the trypsin protein. Equations have been derived which agree quantitatively with the various inactivation experiments.