Spinal cords of sea lamprey larvae were transected at one of two levels: (a) rostral, at the last gill, or (b) caudal, at the cloaca. Following various recovery times, regeneration of the posteriorly projecting giant reticulospinal axons (RAs) was demonstrated by intra-axonal injection of horseradish peroxidase (HRP). Regeneration of axons of anteriorly projecting dorsal cells (DCs) and giant interneurons (GIs) was demonstrated by intrasomatic HRP injection into cells located just below the transection scar. After 40 days of recovery, 55% of proximally transected RAs (rostral cut) regenerated at least as far as the center of the scar, whereas only 15% of distally transected RAs (caudal cut) did so. Maximum distance of regeneration was 5.3 mm beyond the scar for proximally transected RAs but only 38 u for distally transected RAs. Proximally transected RAs also branched more profusely than distally transected ones. These data (when combined with others in the literature) suggest that the regenerative capacity of RAs may decrease with distance of axotomy from the cell body. Distance of regeneration and degree of branching of proximally transected RAs peaked between 40 and 100 days. Thereafter, there appeared to be a tendency toward neurite retraction. Of axotomized GIs, 76% regenerated anteriorly at least as far as the center of a caudal transection scar (GIs are located only in the caudal part of the cord). The maximum distance of regeneration was 1.3 mm beyond the scar. Of DC axons, 56% regenerated anteriorly at least as far as the transection site. The maximum distance was 1.1 mm beyond the scar. DCs located just below a caudal transection regenerated at least as well as those located below a rostral transection. Axonal regeneration was also demonstrated for a few lateral cells, edge cells, and crossed caudally projecting interneurons.