We examined the effects that ultra-brief, intense, light flashes have on the rat circadian clock, the suprachiasmatic nucleus of the hypothalamus (SCN). We found that as few as five intense flashes, each 10-micros in duration (1 per s), can produce both phase shifts in free-running activity rhythms and Fos expression in the SCN in rats kept in constant darkness. After pre-exposure to such flashes, phase shifts in response to a continuous light pulse delivered 2 h later were potentiated, but Fos expression in the SCN was decreased as following pre-exposure to continuous light. These results show that flashes induce behavioral and cellular effects indicative of clock resetting similar to those induced by light stimuli of longer duration. Extremely brief but intense, light stimuli may be much more important to clock resetting than had been previously known.