Rats raised from birth on a 7 A.M.:7 P.M. light cycle exhibited a circadian burst of shedding 1 1/2 hr after the scheduled onset of light. This rhythmic event persisted even after the animals had been placed in total darkness for up to 2 weeks. When animals were placed in constant light (LL) for more than 24 hr, the shedding peak was abolished. A period of darkness of at least 2 hr, ending at the normal time for light onset, was necessary for the production of a shedding burst. When animals were placed in altered lighting cycles, the shedding rhythm shifted accordingly, although the speed of this shift was greatest with large advances in the light cycle and slowest with small delays in the light cycle. Animals exposed to LL and then returned to the dark exhibited the original shedding rhythm, a phenomenon not affected when the optic nerves were severed. A shedding peak could be induced at a noncircadian time by exposing the animals to more than 20 hr of LL followed by 2 hr of darkness and 1 1/2 hr of light, although the magnitude of the shedding was related to the circadian time.