Albino rats were raised in cyclic lighting and subjected to either constant light inhibition of the shedding peak or a 10 hr phase advance in the lighting schedule, with lighting levels of varied intensity. All responses followed sigmoid curves of shedding vs. intensity. For constant light inhibition the curve had a shallow slope with a 50% peak at 0.85 ft-cd. Shedding after a phase shift showed a steep curve with a 50% peak at 0.3 ft-cd when animals were observed at 9:30 P. M. and a shallow slope with a 50% peak at 0.003 ft-cd when observed at 7:30 A. M. It is concluded that the mechanisms of shedding inhibition and phase shifts in the shedding rhythm differ with respect to the role of light.