During the last years, evidence has been accumulated indicating that the annual cycles of physiological and behavioral functions in animals are often preprogrammed as endogenous circannual rhythms. Under seasonally constant environmental conditions these rhythms persist for several cycles with periods deviating from year, whereas under natural conditions they are usually synchronized by seasonal changes in the environment, particularly that of photoperiod. Hence, the annual rhythm of photoperiod constitutes a Zeitgeber for the endogenous rhythmicity rather than providing causal stimuli that release particular activities at particular times of the year. On the basis of this new concept several peculiarities of annual biological rhythms under the influence of photoperiodic cycles with different properties can be better understood, at least at a formal level.