Vasopressinergic projections from the bed nucleus of the stria terminalis (BNST) are sexually dimorphic, with males having denser projections and more vasopressin cells than females in most species studied. This sex difference is established and maintained by steroid hormones. This system appears ideal to study phenotypic differentiation of neuronal phenotype since, in rats, the vasopressin expressing neurons in the BNST are a subset of a larger group of neurons that express the neuropeptide galanin, which themselves don’t differ in number. Sexual differentiation of vasopressin innervation therefore appears to depend on galanin cells deciding whether or not to express vasopressin. Studying the cellular and molecular mechanisms underlying differentiation of vasopressin expression is complex, however, as the onset of vasopressin expression coincides or even lags behind the time that gonadal steroids induce its differentiation. Galanin expression, however, starts before this time and therefore may be used as a marker for potential vasopressin cells. Genetically manipulated mouse models would be ideal to study cellular and molecular mechanisms underlying differentiation of vasopressin expression. Because it is unknown whether galanin and vasopressin are co-expressed in mice, however, we used dual-label immunofluorescence to co-localize vasopressin and galanin in the mouse brain. We found that practically all vasopressin cells in the BNST co-express galanin, but only a subset of galanin expresses vasopressin. This co-localization appears to be specific for BNST cells as we identified cells that express only vasopressin in the paraventricular nucleus and cells that express only galanin in the the preoptic area. The results from this study indicate that galanin can be used as a marker to study differentiation of vasopressin expression in mice.