Environmental light entrains activity of the suprachiasmatic nucleus (SCN), the mammalian circadian pacemaker. Using Fos as marker of neuronal activation, we investigated whether chronic systemic endotoxin administration can alter SCN responsiveness to light after different time intervals. Mice were treated i.p. with lipopolysaccharide for 60 days, and exposed to photic stimulation 7 or 30 days later. Light-induced Fos immunosignal in the SCN was highly significantly decreased versus controls at 7 days, and returned to control levels at 30 days. The data show that SCN function is altered by chronic inflammation and that this effect is reversible over time.