Variable Rate Application (VRA) based on Site Specific Crop Management (SSCM) is an effective approach for optimizing the application of agricultural inputs and preventing the undesirable environmental impacts. Design, construction and evaluation of a Map-Based VRA System was the main objective followed in this research. For variable rate application based on the generated map, a Direct Injection (DI) system was designed and constructed. This system was based on GPS data for positioning of sprayer, comparing GPS data with management map data, measurement of velocity and finally injection of the active ingredient into a carrier fluid using solenoid injectors proportionate to any management zone on the digital management map. The most important factor for evaluation of the developed DI system was system response (delay) time. Electrical conductivity was employed to assess the delay time. Independent variables including duty cycle, travel speed, carrier pressure, active ingredient pressure, active ingredient rate, as based on digital management map, and injection position were varied during the tests. Each test was implemented in three replications. Analysis of Variance and Duncans multiple range test were utilized to analyze the data. A factorial design of a completely randomized block was used for an evaluation of the interactions of various factors. The results revealed that response time depended significantly on carrier fluid pressure and injection position of the active ingredient.