This study in birds provides anatomical, immunohistochemical, and hodological data on a prosencephalic region in which the nomenclature is still a matter of discussion. In quail, this region is located just dorsal to the anterior commissure and extends from the level of the medial part of the preoptic area at its most rostral end to the caudal aspects of the nucleus preopticus medialis. At this caudal level, it reaches its maximal elongation and extends from the ventral tip of the lateral ventricles to the dorsolateral aspects of the paraventricular nucleus. This area contains aromatase-immunoreactive cells and a sexually dimorphic population of small, vasotocinergic neurons. The Nissl staining of adjacent sections revealed the presence of a cluster of intensely stained cells outlining the same region delineated by the vasotocin-immunoreactive structures. Cytoarchitectonic, immunohistochemical, and in situ hybridization data support the notion that this area is similar and is probably homologous to the medial part of the nucleus of the stria terminalis of the mammalian brain. The present data provide a clear definition of this nucleus in quail: They show for the first time the presence of sexually dimorphic vasotocinergic neurons in this region of the quail brain and provide the first detailed description of this region in an avian species.