A relationship between the operation time of a catalyst for NH3 synthesis and the NH3 prodn. cost and profits was studied. A method for detn. of the optimum operation time was described. The detn. of the reactor efficiency was based on the known course of the ferrous catalyst deactivation as established from the data collected during 12 year long industrial operation. The optimum time of the catalyst operation was 4.5 years. After 9 year long operation, the profits from the NH3 production were decreased by 1.5%.