The action of the generated ammonium carbonate in retarding the action of urease is due to the alkalinity of the carbonate. When the solution is kept neutral by a proper phosphate mixture the products have no effect on the velocity of the reaction. Elimination of the effect of the products makes urease a particularly favorable enzyme with which to study the reaction between enzyme and substrate. The results indicate that the action consists of two successive reactions: combination of enzyme and substrate in definite proportions; and decomposition of the compound, the urea being thrown off as ammonium carbonate; each of the two reactions consuming a definite portion of the total time. Formulation of these relations leads to the equation representing the time required for the decomposition of x amount of the initial substrate amount, a; c is a constant representing the velocity of combination of enzyme and substrate, d representing the velocity of decomposition of the complex. The values of c and d can be d...