Several studies have indicated the presence of the neuropeptide vasopressin in the retina, although the origin of the peptide was unknown. We (1) and others (2) have recently discovered vasopressin synthesising cells in the retina itself. Using immunohistochemical and tracing studies, we now show that there are two distinct populations of vasopressin expressing cell types, one in the inner nuclear layer and one in the retinal ganglion cell (RGC) layer. These cells do not express either melanopsin or PACAP and thus are not part of the intrinsically photosensitive RGC population. Intravitreal injections of a rAAV-expressing Venus (under the control of the vasopressin promoter) show that the majority of vasopressin-RGCs project to the ventro-lateral core part of the suprachiasmatic nucleus (SCN). Microdialysis data show an increase in intra-SCN vasopressin concentration in response to electrical stimulation of the optic nerve. Furthermore, in response to optic nerve stimulation some SCN neurons showed a long excitatory response which was blocked by icv injection of a vasopressin V1 receptor antagonist. We are currently studying the physiological significance of this projection; our data suggest that activation of vasopressin-RGC may mediate light cues by releasing vasopressin into the SCN to excite SCN neurons, altering SCN activity and thus regulating neuroendocrine hormonal rhythms.