The paraventricular nucleus of the hypothalamus (PVH) is innervated by a variety of types of neuropeptide-immunoreactive fibers. The cells of origin for many of these inputs are not known. In the present study, the combined retrograde fluorescence-immunofluorescence method was used to determine the cells of origin for neurotensin-, corticotropin-releasing factor-, brain natriuretic peptide-, somatostatin-, and met-enkephalin-like immunoreactive (-ir) fibers in the PVH. After injections of the fluorescent tracer Fluorogold into the PVH, the pattern of retrograde labeling was as previously reported (Sawchenko and Swanson, 1983, J. Comp. Neurol. 218:121-144; McKellar and Loewy, 1981, Brain Res. 217:351-357). The distribution of each type of double-labeled neuron was unique. Retrogradely labeled enkephalin-ir neurons were concentrated in two locations: the ventral part of the lateral septal nucleus and the lateral anterior nucleus within the AHA. A small cluster of corticotropin-releasing factor-ir neurons in the ventral lateral subnucleus of the bed nucleus of the stria terminalis were retrogradely labeled. Notable concentrations of somatostatin-ir double-labeled neurons were found in the ventral part of the lateral hypothalamic area and the medial part of the arcuate hypothalamic nucleus. Neurotensin-ir double-labeled neurons were most numerous in the anteroventral periventricular nucleus and in the retrochiasmatic area. Many brain natiuretic peptide-ir neurons in the tuberomammillary nucleus of the hypothalamus and in the pedunculopontine and laterodorsal tegmental nuclei were retrogradely labeled. The specificity of these chemically defined projections helps lay the groundwork for examining the functional organization of PVH afferents.