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New equipments for variable rate site specific chemical application are being developed for precision farming. These equipments are able to apply the correct rate of chemicals in a site specific manner. The performance of a chemical injection metering system depends mainly on the time of response of the system. The purpose of this work was to develop and apply a methodology for determination of the time of response of a chemical injection metering system mounted on a self-propelled sprayer. This self-propelled sprayer used a Mid-Tech TASC-6600 chemical injection metering system. An AG-132 Trimble DGPS model was used for the vehicle positioning. The research was conducted in a field cultivated with pear millet (Pennisetum americanum) in the stage of development of 8-10 leaves. This field was subdivided into rectangles of 21x25 m with the help of the Farm Site Mate GIS software. A prescription map was created for an herbicide blend of diurom and paraquat applied on two consecutives rectangles in this field. Five points were marked in the path of the self-propelled sprayer corresponding to the initial point of path, the initial point of chemical injection in the system, the initial point of chemical application in the field, the final point of chemical application in the field and the final point of the path. Then, the herbicides application was performed on a planned path with a constant speed of 2.78 m/s and with a theoretical time of response of 25 s. The map of the plants controlled by the action of the herbicides was created. The GIS software SSToolbox was used to overlap the maps and to calculate the time of response for the sprayer boom length, in the field conditions. The time of response of the system was 28 s for 95% of the boom length.