The development of the spinal projections of the nucleus paraventricularis hypothalami (PVH) was studied by injecting wheatgerm-agglutinated horseradish peroxidase into the upper thoracic spinal cord of fetal rats at developmental ages ranging from embryonic day 16 (E16) to E21, and into the thoracic spinal cord of rat pups and adult rats. At E18 the first retrogradely labeled cells are seen in the PVH. These cells are located dorsally in the PVH, adjacent to the ventricular matrix. At E20 a more ventrally located group of retrogradely labeled cells appears. This ventral group joins caudally with the dorsal group of labeled cells. The dorsal group of labeled cells is closely packed, while the ventral group of labeled cells is intermingled with unlabeled cells. After E20 this spatial configuration remains essentially constant. Therefore descending projections from all spinal projecting cell groups in the PVH have reached lower cervical spinal cord levels at E20.