When rodents are placed in a fluctuating light cycle, i.e. a 24-hour cycle without a discrete dawn or dusk and without spectral shifts, they are able to synchronize their circadian rhythms with (entrain to) this cycle, even when, in the case of some ground squirrels, the amplitude of the cycle gets down to a ratio of less than 2-5. In light cycles of so low an amplitude that the rodents do not entrain, they nevertheless do systematically change their circadian frequency concomitant with the light intensity experienced during their time of activity each day, as that time on successive days scans the 24-hour light cycle; this phenomenon is called oscillatory free runs. These observations suggest that the absolute light intensity in each light cycle speeds up or slows down the normal rate of 360 Β° of angular displacement in each circadian cycle. A velocity response curve is proposed in which the speeding up or slowing down would occur only during certain hours of competency in the circadian cycle, the rate of change being concomitant with (1) the differing degree of sensitivity or response to light at various times during those competent hours, and (2) the absolute light intensity.