Since July 1986, cryoanalgesia has been used for the relief of pain after thoracotomy as a routine method of postoperative pain control in our unit. A clinical study of cryoanalgesia showed that it was effective in controling postthoracotomy pain. There is little doubt that freezing provides an effective method of producing prolonged interruption of nerve conduction. This study showed the ultrastructual changes induced by freezing the intercostal serve of mongrel dogs. The earliest changes were dissociation of leaflets of the myelin seath and contraction of the axon followed by the degeneration of both elements. The second change was the absorption of degenerated myelin followed by the regeneration of nerves which were observed at a time when some degenerative changes were still progressing. No relationship has been found between nerve fiber size and susceptibility to freezing. There was a relationship between the duration of freezing and the time required for regeneration of the nerve. Although nerve function was not studied in this experiment, the structual changes were in accord with the clinical findiing of reduced sensitivity during and immediately following cryoanalgesia and also with the recovery of function within a few months of freezing.