The purpose of this investigation was to determine if the cyclic variations in the amount of arrestin mRNA that occur in the mouse retina during the light/dark cycle are controlled by light, a circadian clock, or both. In this study, an experimental paradigm was designed and used to distinguish among these molecular mechanisms. The data show that the initiation and the maintenance of an increased rate of syntheses of arrestin mRNA requires light and is not dependent on the time of day. Therefore the changes in transcriptional activity of the mouse arrestin gene are controlled by light, independent of circadian clock. The mechanism for this light-dependent regulation of gene transcription in the retina is distinct from the light-dependent increases in mRNA concentrations of early response genes which occur in the suprachiasmatic nucleus and a comparative analysis of such changes in these different regions of the CNS may lead to the identification of the molecular relationship between light- and circadian-dependent gene regulation.