This paper presents analytical solutions and interpretation methods for transient testing of water injection wells in a commingled multilayer oil reservoir without crossflow in the formation. The solutions account for saturation distributions that are established in the layers due to relative permeability effects. These solutions show that pressure falloff data usually provide averaged waterflooding parameters for a layered reservoir, and that the properties of individual layers may not be obtained uniquely from such data. An alternate testing and interpretation technique based on simultaneous acquisition of transient downhole layer flowrates and bottomhole pressure is presented. The sensitivity of the interpretation method to errors in flowrate measurements is discussed.