Signaling via the p42/p44 mitogen activated protein kinase (MAPK) pathway has been implicated as an intermediate event coupling light to entrainment of the mammalian circadian clock located in the suprachiasmatic nucleus (SCN). To examine how photic input dynamically regulates the activation state of the MAPK pathway, we monitored extracellular signal-regulated kinase (ERK) activation using different light stimulus paradigms. Compared with control animals not exposed to light, a 15 min light exposure during the early night triggered a marked increase in ERK activation and the translocation of ERK from the cytosol to the nucleus. ERK activation peaked 15 min after light onset, then returned to near basal levels within ∼45 min. The MAPK pathway could be reactivated multiple times by light pulses spaced 45 min apart, indicating that the MAPK cascade rapidly resets and resolves individual light pulses into discrete signaling events. Under conditions of constant light (120 min), the time course for ERK activation, nuclear translocation, and inactivation was similar to the time course observed after a 15-min light treatment. The parallels between the ERK inactivation profiles elicited by a 15 and a 120 min light exposure suggest that SCN cells contain a MAPK pathway signal-termination mechanism that limits the duration of pathway activation. This concept was supported by the observation that the small G protein Ras, a regulator of the MAPK pathway, remained in the active, GTP-bound, state under conditions of constant light (120-min duration), indicating that photic information was relayed to the SCN and that SCN cells maintained their responsiveness for the duration of the light treatment. The SCN expressed both nuclear MAPK phosphatases (MKP-1 and MKP-2) and the cytosolic MAPK phosphatase Mkp-3, thus providing mechanisms by which light-induced ERK activation is terminated. Collectively, these observations provide important new information regarding the regulation of the MAPK cascade, a signaling intermediate that couples light to resetting of the SCN clock.