The distribution of vasopressin (VP) and oxytocin (OT) neurons in the rat supraoptic (SON), paraventricular (PVN), and accessory magnocellular (AMN) nuclei was studied by localizing both peptides on the same section with a double immunocytochemical staining procedure employing specific monoclonal antibodies (MAB). This procedure allows us to visualize the distribution of one cell type relative to the other. In the rostral SON, VP cells lie dorsal and medial to the OT cells. Near the mid-point of the nucleus along its rostral-caudal length, there is a transition zone in which the two cell types are mixed. Proceeding caudalward, the relative locations of OT and VP cells are exchanged so that most of VP cells are located in the ventral and medial sector of the nucleus, whereas the OT cells are situated dorsal and lateral. However, there is no absolute segregation of the two types of cells anywhere in the nucleus. In the anterior part of the PVN a rostral group (rPVN) of cells composed of a medial portion and a lateral wing can be recognized. Nearly all of the cells in the rPVN are oxytocin-containing. The rPVN is separated from the next group, the middle PVN (mPVN), by a cell poor zone of about 100–150 µm. The mPVN contains both OT and VP neurons. As one proceeds caudally, the OT cells extend in the rostrocaudal direction from an anterior and ventromedial location, forming a shell around a core of VP neurons. In the most caudal PVN (cPVN), a triangular cell group characterized by fusiform cells with long-beaded processes can be distinguished from the more rounded cells of the remaining PVN. Many fusiform cells in the cPVN appear to send their axons to the posterior perifornical nucleus and the nucleus of the medial forebrain bundle. Other fusiform cells of the cPVN are oriented in a rostral-caudal plane and are situated more medially in this subdivision. The dendrites of these cells project into the mPVN while their posterior processes, most of which also appear to be dendrites, project caudally along a medial rou