We have summarized here recent evidence that clarifies the cellular organization and connections of the paraventricular nucleus of the hypothalamus (PVH) in the rat. The nucleus consists of a magnocellular division, with three distinct parts, and a parvocellular division with five distinct parts. Most neurons in the magnocellular division contain either oxytocin or vasopressin, and project to the posterior lobe of the pituitary gland. Separate cell populations centered in the parvocellular division give rise to projections to the median eminence, or to the brain stem and spinal cord including the intermediolateral column; some cells project both to the dorsal vagal complex and to the spinal cord. Cells with long descending projections may contain either oxytocin, vasopressin, somatostatin, or dopamine, although the biochemical specificity of most such neurons has not been determined. Noradrenergic fibers are found preferentially within those parts of the magnocellular division that are predominantly vasopressinergic. The parvocellular division is innervated by adrenergic as well as noradrenergic fibers from the brain stem, and by fibers from the dorsal vagal complex and the parabrachial nucleus. The bed nucleus of the stria terminalis and adjacent parts of the hypothalamus also innervate the PVH. The evidence indicates that subpopulations of neurons in the PVH are directly related to autonomic and neuroendocrine effector mechanisms, and suggest that the nucleus plays an important role in the regulation of visceral responses in the periphery and in the CNS itself.