The degree to which mice react to light was measured by comparing open-field activities of mice produced by backcrosses involving A/J and C57BL/6J parental strains under normal white light and under dim red light. This reactivity was markedly enhanced, for pigmented mice, by rearing the animals in the dark: the reactivity of albino mice was unaffected by dark-rearing. Total activity, but not reactivity to light, was found to be influenced by the genotype of the mother, the offspring of hybrid mothers being less active than the offspring of inbred mothers. The findings are discussed in terms of the previously advanced hypothesis that retinal melanin may serve as a photochrome.