The outgrowth of regenerating frog sciatic sensory axons after a crush lesion was measured from the distribution of axonally transported radioactive proteins. Five days after a single test crush the regeneration distance was 3.6 +/- 0.3 mm. If the nerve was subjected to a conditioning lesion 15 mm distal to and 5, 10 or 17 days prior to the test crush, there was an increased outgrowth distance (5.1 +/- 0.2 mm) 5 days after the test crush when the conditioning interval was 10 days. Prolongation of the time interval between the two lesions to 17 days did not significantly change the enhanced outgrowth. In contrast, if the conditioning and test lesions were applied at the same site, the regeneration distance 5 days after the test crush was already significantly increased after a conditioning interval of 5 days (5.9 +/- 0.5 mm). It was further enhanced after an interval of 10 days (7.8 +/- 0.5 mm) and yet further after 17 days (9.2 +/- 0.5 mm). When the conditioning and test lesions were superimposed, the regeneration occurred through a region of the nerve that was pre-degenerated due to the conditioning lesion. The positive correlation between the degree of degeneration and the outgrowth distance supports the theory that environmental factors contribute to the regenerating ability of peripheral nerves.