The number and topographic distribution of the profiles of degenerating primary afferent fibres were determined within the rat dorsal column 3-4 weeks after division of the lumbar and S2 dorsal roots. The degenerating fibres were identified in toluidine blue-stained 1 micron transverse sections taken at different spinal levels, and their positions were marked with the aid of a drawing tube. Fibres entered the dorsal column at its lateral margin and sent projections rostrally and caudally. Fibres ascending the column were displaced medially in an orderly progression as the fibres of more rostral roots entered the cord. Most ascending fibres were lost from the dorsal columns within 2-3 segments of their site of entry, with only 15%, on average, reaching cervical levels. The descending fibres maintained a less organised topographic distribution, and typically only 3% of fibres entering the dorsal column descended two segments from their site of entry.