The regeneration rate of peripheral nerve axons containing substance P-like immunoreactivity (SPLI) was measured in rat sciatic nerve by radioimmunoassay of SPLI in nerve segments 2, 4 and 6 days after a test lesion made by briefly crushing the nerve at the hip. The regeneration rate of the fastest growing sensory axons was also measured in the same nerves using the pinch-reflex procedure. Three groups of animals were compared: group S, which received only the single test lesion, had regeneration rates of 3.57 +/- 0.26 (S.E.) mm/day for SPLI-containing axons and 3.53 +/- 0.14 mm/day for the fastest growing sensory axons. Group A/H, which received a conditioning lesion on the tibial nerve at the ankle 7 days prior to the test lesion at the hip, had a regeneration rate for SPLI-containing axons which was not significantly different from group S, of 3.35 +/- 0.17 mm/day. However, the regeneration rate for the sensory axons was significantly increased to 4.60 +/- 0.23 mm/day. Group H/H, which received both conditioning and test lesions at the hip, once again separated by 7 days, showed a significant increase in regeneration rate of SPLI-containing axons to 5.50 +/- 0.33 mm/day and a further increase over group A/H in the regeneration rate of sensory axons to 6.70 +/- 0.25 mm/day. We conclude that the small-diameter, unmyelinated axons containing SPLI in peripheral nerve normally regenerate at the same rate as the fastest growing sensory axons.(ABSTRACT TRUNCATED AT 250 WORDS)