Methods that allow specific manipulations of the mouse genome have made it possible to alter specific aspects of photoreceptor function within the mouse retina. Mice with photoreceptors that have altered photosensitivities and altered photoresponse kinetics are now available. Methods are needed that can show how those perturbations in photoreceptor response characteristics translate into perturbations in visual sensitivity and perception. We have adapted a previously described method to evaluate visual threshold, spatial discrimination and temporal discrimination in mice swimming in a water maze. In this report we describe the sensitivities of rod-mediated and cone-mediated vision using GNAT1−/− and GNAT2−/− mice. Cone-mediated vision is ∼10 000 times less sensitive than rod-mediated vision in mice. We also demonstrate that mice can distinguish striped from solid objects in the water maze and that they can distinguish flickering from continuous illumination.