The lactate dehydrogenase catalysed reaction shows lag phase. This lag phase is easy for explanation if the consecution of first and second order equilibrium reactions were assumed for calculation of pyruvate trace concentration. The same explanation was accepted for calculation of significant pyruvate concentration. For calculation significant pyruvate concentration the consecution of simple Michaelis - Menten type and second order reactions was assumed. The exact solution of reaction rate equations system for this complex reaction was counted.