A 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.