Following a 'test lesion' (crush) of the optic nerve in goldfish, histological study of axons in silver-stained sections showed that outgrowth of the leading axons began after an initial delay of 4.3 days and proceeded at 0.34 +/- 0.03 mm/day. When a 'conditioning lesion' (crush at the same site) preceded the testing lesion by 2 weeks, the initial delay was 2.5 days and the outgrowth rate was 0.74 +/- 0.13 mm/day (P less than 0.01). Two additional methods, utilizing intraocular injections of tritiated proline or fucose to label axonally transported proteins, were used to examine the outgrowth of leading optic axons. (a) Measurement of the distances reached by labeled axons in the nerve at 6 and 10 days after a testing lesion alone yielded an initial delay of 4.6 days and an outgrowth rate of 0.41 +/- 0.04 mm/day. However, when a conditioning lesion preceded the testing lesion, labeled optic axons were already found to have reached the optic tectum by 10 days after the testing lesion, indicating an outgrowth rate in excess of 0.64 mm/day. (b) Determination of the times at which labeled axons arrived at the optic tectum showed that the outgrowth rate after a testing lesion along was 0.40 mm/day whereas when the testing lesion was preceded by a conditioning lesion it was 0.74 mm/day. Thus, as a result of a conditioning lesion the initial delay was reduced by nearly half and the outgrowth rate was nearly doubled.