The distributions of descending and ascending spinal projection neurons (i.e., spinal neurons with moderate to long axons) were compared in normal larval lamprey and in animals that had recovered for 8 weeks following a complete spinal cord transection at 50% body length (BL, normalized distance from the anterior head). In normal animals, application of HRP to the spinal cord at 60% BL (40% BL) labeled an average of 713.8 +/- 143.2 descending spinal projection neurons (718.4 +/- 108.0 ascending spinal projection neurons) along the rostral (caudal) spinal cord, most of which were unidentified neurons. Some of these neurons project for at least approximately 50-60 spinal cord segments (approximately 36-47 mm in animals with an average length of approximately 90 mm used in the present study). At 8 weeks posttransection, the numbers of HRP-labeled descending or ascending spinal neurons that extended their axons through the transection were about 40% of those in similar areas of the spinal cord in normal animals. Thus, in larval lamprey, axonal regeneration of descending and ascending spinal projection neurons is incomplete, similar to that found for descending brain neurons. The majority of restored projections were from unidentified spinal neurons that have not been documented previously. In contrast to results from several other lower vertebrates, in the lamprey ascending spinal neurons exhibited substantial axonal regeneration. Identified descending spinal neurons, such as lateral interneurons and crossed contralateral interneurons, and identified ascending spinal neurons, such as giant interneurons and edge cells, regenerated their axons at least 9 mm beyond the transection site in animals with an average length of approximately 90 mm, which is appreciably farther than previously reported. In contrast, most dorsal cells, which are centrally located sensory neurons, exhibited very little axonal regeneration.