AbstractA composite confocal photomicrograph showing the organization an topographic relations of specific neuronal populations in the hypothalamic paraventricular nucleus (PVH) of a C57BL/6malemouse. One population of PVH neurons (presumptive neuroendocrine) was identified by immunocytochemical detection of the retrograde tracer Fluorogold (FG; green) following its injection in to the tail vein; a second population of PVH neurons (presumptive preautonomic) was back‐labeled following upper thoracic level spinal cord injection of the retrograde tracer Fast Blue (FB; blue). Subsequently, distinct populations of oxytocin (OXY; red)‐ and vasopressin (VAS; magenta)‐ expressing neurons were identified immunocytochemically. At this PVH level neurons labeled only with FG (green) include numerous presumptive parvicellular neuroendocrine neurons. None of the FB (blue) back‐labeled neurons were immunopositive for OXY or VAS at this PVH level; however all of the OXY‐ and VAS immunopositive neurons were back‐labeled with FG, although the labeling intensity varied. Thus, OXY‐expressing FG labeled neurons appear red/yellow, whereas VAS‐expressing FG labeled neurons appear magenta/white. The Journal of Comparative Neurology, Volume 520, Number 1, pages 6–33.