Almost nothing is known about brightness discrimination in animals and how this ability relates to their lifestyles. As arrhythmic visual generalists, three dogs, a German shepherd and two Belgian shepherds, were tested on their ability to discriminate brightness using a series of 30 shades of grey varying from white to black. The dogs were trained to discriminate between different shades of grey in a simultaneous two-choice situation. Weber’s law can be correlated to their ability to discriminate brightness differences with a calculated Weber fraction of 0.22 for the German shepherd and 0.27 for the Belgian shepherds. Thus brightness discrimination in dogs is about 2 times worse than in humans, a diurnal species.