The invention relates to a process for controlling the injection of urea in a selective catalytic reduction, known as CR, system for treating nitrogen oxides, intended to be installed in the exhaust line (6) of a motor vehicle (1) engine (2), wherein the treatment comprises chemically reducing, in a catalyst (3), known as SCR catalyst, the nitrogen oxides by adding ammonia contained in urea, the process comprising the following steps: —the temperature of the gases in the exhaust line (6) of the engine (2), upstream of the SCR catalyst (3), is measured, —if the temperature measured is above a predetermined minimum value, called minimum injection temperature, an order is given to inject urea (5), —if the temperature measured is below the minimum injection temperature, the following substeps are carried out: —a mass of ammonia stored in the SCR catalyst (3) is determined, —the amount of ammonia required to obtain a nitrogen oxide conversion greater than a predetermined value is determined, if the mass of ammonia stored in the catalyst is below this required amount, an order is given to activate a method of heating the exhaust gases, and to inject urea into the system.