The neurosecretory hypothalamo-hindbrain connection of lower vertebrates is also present in the pigeon and the rat. Using an immunohistochemical technique and the pseudoisocyanin technique these fibers could be seen following known pathways through the mesencephalic reticular formation to the hindbrain. Here the fibers enter the region of the dorsal column nuclei. The neurophysin vesicles demonstrable by EM in these fibers are identical with the well known neurophysin vesicles which are present in the neurohypophysis of investigated species. The content of the vesicles is indeed neurophysin. The terminals of the fibers from neuro-neuronal synapses.