In a previous study, we showed that the visual ON response recorded from the superior colliculus is absent in the metabotropic glutamate receptor subtype 6 (mGluR6) knockout mice. However, these mutant mice can respond to some visual inputs and are able to learn a light-dark discrimination task. Here, we systematically investigated the possibility that some ON responses could remain in the mutant mice. Recording light evoked field potentials from the superior colliculus of mGluR6-deficient mice at scotopic and mesopic backgrounds, we found a characteristic ON response that differs from the ON response of wild-type mice. The ON responses recorded from the mutant mice appeared with a latency (200-300 ms) longer than that of the wild-type mice (50-100 ms) and amplitudes of this late ON response decreased upon increase of the light intensity in most cases examined. The late ON response may contribute to the apparent normal behavior and some physiological responses to light stimuli in mGluR6-deficient mice.