As one of a series of studies of the ascending spinal cord pathways that might be involved in nociception in the rat, we have examined the projection to the dorsal column nuclei that originates in the spinal cord dorsal horn using the retrograde transport of horseradish peroxidase (HRP). This projection in other animals has been called the postsynaptic dorsal column (PSDC) pathway. Small iontophoretic injections of HRP into the cuneate nucleus (CN) labeled more than 350 neurons in alternate sections within the ipsilateral gray matter of segments C6-8. Fewer than 25 neurons were labeled in L4-6 by injections into CN. Injections of HRP confined to the gracile nucleus (GN) labeled more than 200 neurons within a narrow band extending across the ipsilateral dorsal horn subjacent to substantia gelatinosa of L4-6. Fewer than 10 cells were labeled in C6-8 by such injections. Labeling in lumbar neurons following injections into GN was prevented by transection of the dorsal columns at T10, T8, or C2. Thus, neurons labeled by such injections ascend entirely within the dorsal columns. Lesions of the dorsal columns in C2 reduced the number of labeled neurons in the cervical cord following CN injections by approximately 90%. Combined lesions of the dorsal columns and ipsilateral dorsal lateral funiculus (DLF) reduced the number of cells labeled in C6-8 by approximately 98%. Thus, the majority of labeled neurons in the cervical enlargement project to CN via the dorsal columns; a small secondary component of the cervical projection to CN appears to ascend within the DLF. To compare the relative sizes of the projections to the dorsal column nuclei from PSDC neurons and dorsal root ganglion cells (DRG), labeled neurons were counted in the gray matter of the cervical and lumbar enlargements and the corresponding DRG. In the four animals so examined, PSDC neurons constituted over 38% of the neurons that projected to CN and approximately 30% of the cells that projected to GN. These findings indicate that the PSDC projection of the rat is capable of providing a large somatotopically organized input to the dorsal column nuclei.