A sexual difference in vasotocin (VT) immunoreactivity was observed in the canary brain. The male canary displays denser VT innervation of the lateral septum and more VT-immunoreactive cells are visible in the dorsal diencephalon than in the female canary. This sex difference is the consequence of gonadal influences. Castration of male canaries resulted in a substantial decrease of VT immunoreactivity in these septal and diencephalic cells and fibers. Testosterone administration to castrated males restored the original intensity of fiber and cell staining. Testosterone treatment of female canaries induced strong VT immunostaining in the above regions similar to that in the male canary.