The rate of regeneration of rat sciatic nerve sensory axons was measured using the pinch-reflex test method, and confirmed by studying the transport of labelled protein into the regenerating axons. For nerves receiving a single test crush lesion the rate was 4.02 +/- 0.03 (SE) mm/day. For nerves with a conditioning lesion made at the knee seven days prior to the test lesion at the hip the rate was 5.73 +/- 0.06 mm/day, and for nerves where both conditioning and test lesions were made at the same site (hip or knee) but separated by seven days, the rate was 6.76 +/- 0.04 mm/day, a 68% increase over the normal rate, showing that pre-degeneration of the nerve distal to the site of the test lesion increases the rate of regeneration. It is concluded that the rate of axon regeneration can be influenced by the environment through which the regenerating axons grow.