The effects of application of a chromic catgut suture (conditioning lesion) placed close to the sciatic or tibial nerves on regeneration of the sciatic nerve after a crush lesion (test lesion), that had been induced after an appropriate conditioning interval (two or four weeks) were assessed. The catgut suture induced an inflammatory reaction around the nerve during the four weeks after application of the catgut suture (conditioning interval) but no signs of degeneration of nerve fibres were seen. There was a significant increase in length of outgrowth of sensory nerve fibres as measured by the pinch reflex test when the test lesion was applied after two and four weeks' exposure to the catgut suture. The rate of regeneration was increased by about 11% when the catgut suture had been applied for four weeks before the test lesion was made. The findings indicate that an inflammatory reaction around a peripheral nerve may act as a conditioning lesion, thereby stimulating regeneration of the nerve.