Recently, a new nuclear receptor subfamily has been identified and referred to as estrogen-related receptors. This new group shares sequence similarity, target genes, co-regulatory proteins, and action sites with the estrogen receptors; however, natural estrogens are not estrogen-related receptors ligands. One of the receptors belonging to this group, estrogen-related receptor beta (ERRbeta), is essential for embryo development and is believed to be involved in estrogen-regulated pathways. In this study, we analyzed the presence of the ERRbeta protein in the mouse brain by means of immunohistochemistry, using a commercial polyclonal antibody against ERRbeta (Sigma, E0156). This study represents the first description dealing with the immunolocalization of ERRbeta in a mammalian brain. Our results revealed numerous ERRbeta immunoreactive fibers in the retinal efferent projections in the brain, which was in agreement with the presence of intense ERRbeta immunoreactivity in the cell bodies and axonal processes of the retinal ganglion cells. In both postnatal and adult brains, ERRbeta immunoreactive fibers were distributed in a pattern which perfectly matched the retinal efferent projections: optic tract, supraoptic commissure, hypothalamic suprachiasmatic nucleus, ventral and dorsal geniculate nuclei, pretectal nuclei, and superior colliculus. Due to reliable, fine, and complete staining of the retinal axons obtained with the anti-ERRbeta antibody (E0156), we suggest that this antibody could be used as a valuable tool for labeling the full retinofugal projections in postnatal or adult brains.