Circadian rhythms in the numbers of alpha- and beta-adrenergic receptors exists in the rat forebrain and hypothalamus. These rhythms are endogenous as they persist in the absence of time cues. The alpha- and beta-receptor rhythms differ both in their shapes (wave form) and in the timing (phase) of the peak numbers. In the course of the year, there are shifts in the timing of both alpha- and beta-receptor peaks. Circadian rhythms in synchronizing both the synaptic events and the behaviors related to them to the photoperiod of the natural environment.