The intergeniculate leaflet (IGL) is an integral part of the adult circadian visual system. It is characterized by the presence of retinal afferents and peptidergic cells projecting via a geniculohypothalamic tract (GHT) to the suprachiasmatic nucleus (SCN), site of the mammalian circadian clock. The adult IGL also contains abundant reactive astrocytes immunoreactive to GFAP. Because glia have a large role in brain development, we examined the ontogeny of the hamster IGL with respect to both glial and neuronal markers. Neuropeptide Y- immunoreactive (NPY-IR) cells destined for the IGL appear on embryonic day 11 (E11) in a matrix of vimentin (VIM)-IR radial glia. Migratory ellipsoid NPY-IR cells with long leading and trailing processes become oriented between the reticular thalamic neuroepithelial lobule, and the developing IGL. Most NPY-IR cells arrive in the IGL by E14 and extend axons ventrally into the GHT. These penetrate the SCN at P3 and arborize to an adult-like stage by P10. A specialized GFAP-IR radial glial path coinciding with the migratory route of NPY-IR cells appears by E14. As early as E15, cells contributing to this path are found displaced away from the ventricle. As the glial path disappears from the maturing brain, the entire length of the IGL becomes filled with GFAP-IR astrocytes. These features are consistent with translocation and transformation of a specialized set of radial glia into IGL astrocytes. The results demonstrate that the IGL is a large, developmentally important, feature of the lateral geniculate complex that is embryologically distinct from adjacent dorsal and ventrolateral geniculate nuclei.