Sex differences in vasopressin and oxytocin expression are helpful in studying the anatomy and function of vasopressin innervation of the brain. They also provide insight in the function of neural sex differences in general. This paper will discuss nature, cause and possible significance of these sex differences, focusing on vasopressin projections from the bed nucleus of the stria terminalis and the medial amygdaloid nucleus, which show some of the most consistently found sex differences among vertebrates.