The distribution of vasopressin or vasotocin immunoreactive cells and fibers in the lateral septum and the bed nucleus of the stria terminalis are sexually dimorphic in many vertebrates including several species of birds examined to date. We examined the vasotocin-like immunoreactivity in the zebra finch brain. Male birds had a higher level of immunoreactive staining in some telencephalic and diencephalic regions. The density of immunostaining increased in the testosterone-treated females to levels typically seen in males. The sexual dimorphism and testosterone dependence of the vasotocin-like immunoreactivity are similar to that found in the canary. Thus this pattern of vasotocin localization and testosterone dependence may be a general feature in brains of passerine songbirds.