An immunocytochemical study of the magnocellular neurosecretory nuclei was performed in the snake Natrix maura and the turtle Mauremys caspica by use of antisera against: (1) a mixture of both bovine neurophysins, (2) bovine oxytocin-neurophysin, (3) arginine vasotocin, and (4) mesotocin. Arginine vasotocin- and mesotocin-immunoreactivities were localized in individual neurons of the supraoptic and paraventricular nuclei, with a distinct pattern of distribution in both species. The same cells appeared to be stained by the anti-oxytocin-neurophysin and antimesotocin sera. The supraoptic nucleus can be subdivided into rostral medial and caudal portions. In N. maura, but not in M. caspica, neurophysin-immunoreactive neurons were found in the retrochiasmatic nucleus. No immunoreactive elements were seen in the suprachiasmatic nucleus of both species after the use of any of the antisera. A dorsolateral aggregation of neurophysin-containing cells, localized over the lateral forebrain bundle, was present in both species. Magnocellular and parvocellular neurophysin-immunoreactive neurons were present in the paraventricular nucleus of both species. In the turtle, the paraventricular neurons were arranged into four distinct layers parallel to the ependyma; these neurons were bipolar with the major axis perpendicular to the ventricle, and many of them projected processes toward the cerebrospinal-fluid compartment. In N. maura a group of large neurons of the paraventricular nucleus was found in a very lateral position. The posterior lobe of the hypophysis and the external zone of the median eminence contained arginine vasotocin- and mesotocin-immunoreactive nerve fibers. The lamina termialis of both species was supplied with a dense bundle of fibers containing immunoreactive neurophysin. Neurophysin-immunoreactive fibers were also present in the septum, some telencephalic regions, including the cortex and the olfactory tubercule, in the paraventricular organ, and the periventricular and periaqueductal gray of the brainstem.