The overall goal is to specify the neuroendocrine bases for mammalian circannual rhythms (CARs) in behavior. All experiments will be conducted on the golden-mantled ground squirrel (Spermophilus lateralis). Aims include: (1) assessing the role of the pineal gland in photic entrainment of CARs; (2) elaborating the ways in which light and melatonin entrain CARs; (3) characterizing the phase response curve of CARs to melatonin and estradiol; (4) determining neural sites at which melatonin and estradiol phase-shift CARs; (5) localizing circannual pacemakers in the brain; (6) assessing sex differences in circannual organization; (7) evaluating the role of the suprachiasmatic nuclei in temporal organization of the hibernation CAR and (8) establishing whether endogenous gonadal hormones prevent torpor and signal the end of the hibernation season. The biological origin of human annual rhythms, including seasonal affective dysfunctions, is generally accepted. The study of CARs in animals remains the major source of information on the subject and holds the best prospect of understanding and manipulating human CARs.