The extent of the retrograde degeneration of corticospinal axons following transection of the spinal cord was studied in rats by labeling corticospinal axons with anterogradely transported horseradish peroxidase injected in the sensorimotor cortex. Axotomized corticospinal axons underwent progressive and continuing retrograde degeneration. In specimens examined 5, 14, 28, and 56 days after trauma, the tips of the transected corticospinal axons were seen to terminate at 181 +/- 80 microns, 977 +/- 203 microns, 1751 +/- 344 microns, and 2559 +/- 466 microns (mean +/- standard deviation), respectively, from the site of transection. The rate of retrograde degeneration varied according to the interval after spinal cord transection, as follows: 36.2 microns/day during the first 5 days; 88.4 microns/day between 5 and 14 days; 55.3 microns/day between 14 and 28 days; and 28.8 microns/day between 28 and 56 days. These findings may serve as useful parameters for the objective assessment of therapeutic modalities in spinal cord injury research.