The effect of diffusion on the reversible enzyme-catalyzed reaction is investigated. The kinetic scheme of the enzyme-catalyzed reaction is considered with a little generalization of Michaelis-Menten mechanism where the last step is taken as reversible. By using a fluctuation theory approach we have considered the relaxation mechanism where the quadratic nonlinearity of the equation of fluctuation around the equilibrium acts as a small perturbation. In three dimensions, the effect of diffusion is reflected through the relaxation kinetics of the reaction as a power law asymptotics, t(-3/2), when the system approaches equilibrium. (C) 2003 American Institute of Physics.