“…For example, serotonergic activity influences repro duction in several ways, including an inhibition of gonado tropic hormone release [21,23], an activity that would be important in fall and winter when gonadal hormones could prevent hibernation [16]. Thus, it is of interest that brain se rotonin content and apparent activity are greater during fall and winter in several hibernatory species [8,18,27,32,34],The seasonal change in daylength is an important regu lator of the annual cycle of reproduction in the golden ham ster, Mesocrieetus auratus. However, an endogenous timer is equally important in that it determines how daylength is interpreted during the year.…”