Neurones of the suprachiasmatic nucleus constitute the mammalian circadian clock which receives photic information via the retino-hypothalamic tract and to some extent non-photic information via the geniculo-hypothalamic tract. The majority of neurones in the geniculo-hypothalamic tract contains neuropeptide Y and both in vitro and in vivo physiological experiments have demonstrated that neuropeptide Y administered directly into the suprachiasmatic nucleus has the capacity to phase-shift the endogenous circadian rhythm of these neurones. The recent cloning of multiple mammalian neuropeptide Y receptors enabled us to perform an in situ hybridization histochemical study identifying expression of distinct neuropeptide Y receptor encoding mRNAs in the suprachiasmatic nucleus. It was seen that Y1 and Y5 receptor mRNA is highly expressed in neurones of the ventrolateral portion of the suprachiasmatic nucleus while neither Y2 nor Y4 receptor mRNA could be detected in the nucleus. These experiments demonstrate that post-synaptic neuropeptide Y mediated events in the suprachiasmatic nucleus are likely to be mediated by either of these receptors.