Immunoreactive neurophysin (Np) is demonstrated by the immunoperoxidase technique not only within neurones of the classical hypothalamo-neurohypophyseal system, but also in extrahypothalamic perikarya and fibers. Np-positive perikarya are found in the triangular nucleus of the septum. Np fibers join the stria terminalis (ST) at the level of the anterior hypothalamus, and course medially in the ST to the central nucleus of the amygdala. Np fibers are found in the brain stem and spinal cord. Fine caliber Np fibers from the suprachiasmatic nucleus ascend to the medial dorsal thalamus and lateral septum. The presence of Np within neurones implies the presence of either vasopressin or oxytocin. Np-positive fibers in extrahypothalamic sites may interact with non-neurosecretory neurones involved in neuroendocrine regulation, or may serve as yet unknown functions.