Retinoic acid (RA) is a powerful morphogen which is critically important to many diverse processes including embryonic development of the central nervous system. RA signalling has also been found in the adult brain, most notably in the regulation of neurogenesis in the dentate gyrus region of the hippocampus (McCaffery et al. 2006). The hypothalamus is a region of the brain important in the maintenance of homeostasis. Feeding behaviours, the sleep/wake cycle, and reproduction are all co-ordinated by nuclei associated with this area of the adult brain (Morris et al. 2012). The hypothalamus is now known to express several of the proteins necessary for RA signalling. In particular, hypothalamic tanycytes lining the third ventricle express the RA synthetic enzyme RALDH2 and the retinoic acid receptors RARα, RARβ, and RARγ. The expression of these proteins in the hypothalamus is regulated by seasonal changes in day length, or photoperiod (Shearer et al. 2010). A diverse array of organisms ranging from single cell bacteria, to plants, and more advanced species including mammals have all developed an endogenous timing system that links behaviour to external light cues. By this means, physiological processes are optimally synchronised with both the solar day and changes in photoperiod throughout the year. The Suprachiasmatic Nucleus (SCN), located in the anterior hypothalamus is the central pacemaker of the circadian clock which is entrained by external light cues to day length. The aim of the present study was to elucidate whether components of the RA signalling pathway are present in the SCN, and whether levels of expression of these proteins are regulated by photoperiod. Adult male F344 photoperiodic rats were exposed to long day (14:10 Light:Dark) or short day (10:14 Light:Dark) light conditions for a duration of 3 months. The SCN was then subject to analysis for the RA synthetic enzymes (RALDHs) and retinoic acid receptors (RARs) by both immunohistochemistry, and quantitative PCR. The present study describes the expression of the RALDHs and RARs by the SCN in the adult rat hypothalamus. The SCN expresses RALDH2, but not RALDH1 or RALDH3, and RARα, RARβ, and RARγ suggesting that it has the ability to both synthesise, and respond to RA signalling. Given the importance of the SCN in regulating physiological responses to seasonal and daily changes in light conditions, RA signaling may be important in the hypothalamus to the generation of the circadian rhythm in the adult rat