In mammals, the suprachiasmatic nuclei (SCN) of the hypothalamus function as the major biological clock. SCN-dependent rhythms of physiology and behavior are regulated by changes in the environmental light cycle. Currently, the second messenger signaling events that couple photic input to clock entrainment have yet to be well characterized. Recent work has revealed that photic stimulation during the night triggers rapid activation of the p42/44 mitogen activated protein kinase (MAPK) pathway in the SCN. The MAPK signal transduction pathway is a potent regulator of numerous classes of transcription factors and has been shown to play a role in certain forms of neuronal plasticity. These observations led us to examine the role of the MAPK pathway in clock entrainment. Here we report that pharmacological disruption of light-induced MAPK pathway activation in the SCN uncouples photic input from clock entrainment, as assessed by locomotor activity phase. In the absence of photic stimulation, transient disruption of MAPK signaling in the SCN did not alter clock-timing properties. We also report that signaling via the Ca(2+)/calmodulin kinase pathway functions upstream of the MAPK pathway, coupling light to activation of the MAPK pathway. Together these results delineate key intracellular signaling events that underlie light-induced clock entrainment.