Durational melatonin signals, cued by the photoperiod and generated by the pineal gland, are processed in the brain to induce seasonally appropriate physiological and behavioural adaptations. The melatonin receptor subtype MEL1a (also known as mt1) appears to regulate seasonal responses. Single label in situ hybridization for MEL1a receptor mRNA revealed labelled cells in several brain regions of Siberian hamsters, including the suprachiasmatic nucleus, the paraventricular nucleus of the thalamus, and the reuniens nucleus of the thalamus. To characterize suprachiasmatic nucleus cells containing MEL1a receptor mRNA, we used 35S-labelled cRNA probes for MEL1a receptor mRNA in combination with digoxigenin-labelled cRNA probes for vasopressin, somatostatin, or orphan retinoid Z receptor β (RZRβ; a putative nuclear melatonin receptor). Cells in the suprachiasmatic nucleus that contained MEL1a receptor mRNA also contained mRNAs for vasopressin and RZRβ, but not for somatostatin. These data suggest that suprachiasmatic nucleus vasopressin cells may respond to melatonin signals, raising the possibility that suprachiasmatic nucleus vasopressin output mediates some of the effects of melatonin on seasonal or circadian responses.