The paraventricular nucleus of the hypothalamus (PVH) contains a prominent collection of varicose atriopeptin-like immunoreactive (APir) fibers. We have used immunohistochemistry and fluorescent retrograde tracers to investigate the origin of these fibers. All parts of the PVH contain APir fibers. The densest collections are found in the periventricular area and in the parvocellular components of the nucleus. Somewhat smaller numbers of fibers are found within the borders of the magnocellular part of the PVH. Following the injection of fluorescent retrograde tracers into the PVH and subsequent immunohistochemical staining, numerous retrogradely labeled APir neurons were observed in the anteroventral periventricular nucleus (AVPV), adjacent to the anteroventral tip of the third ventricle. Smaller groups of retrogradely labeled APir neurons were observed in the ventromedial part of the bed nucleus of the stria terminalis, the pedunculopontine and laterodorsal tegmental nuclei, and the medial part of the nucleus of the solitary tract. The APir projection from the AVPV to the PVH is of particular interest in view of the abundant evidence implicating both regions in the regulation of the fluid and electrolyte balance and blood pressure. AP may serve as a central neuromodulator as well as a circulating hormone in cardiovascular regulation.