Arabidopsis seedlings are genetically endowed with the capability to follow two distinct developmental programs: photomorphogenesis in the light and skotomorphogenesis in darkness. The regulatory protein CONSTITUTIVE PHOTO-MORPHOGENIC1 (COP1) has been postulated to act as a repressor of photomorphogenesis in the dark because loss-of-function mutations of COP1 result in dark-grown seedlings phenocopying the light-grown wild-type seedlings. In this study, we tested this working model by overexpressing COP1 in the plant and examining its inhibitory effects on photomorphogenic development. Stable transgenic Arabidopsis lines overexpressing COP1 were generated through Agrobacterium-mediated transformation. Overexpression was achieved using either the strong cauliflower mosaic virus 35S RNA promoter or additional copies of the wild-type gene. Analysis of these transgenic lines demonstrated that higher levels of COP1 can inhibit aspects of photomorphogenic seedling development mediated by either phytochromes or a blue light receptor, and the extent of inhibition correlated quantitatively with the vivo COP1 levels. This result provides direct evidence that COP1 acts as a molecular repressor of photomorphogenic development and that multiple photoreceptors can independently mediate the light inactivation of COP1. It also suggests that a controlled inactivation of COP1 may provide a basis for the ability of plants to respond quantitatively to changing light signals, such as fluence rate and photoperiod.