The cat spinal dorsal column was irradiated unilaterally with argon laser (wave length 497nm, power output 200mW, focus diameter 2mm) for every ten seconds. The somatosensory evoked potentials (S. E. P.) were recorded for neurophysiological monitor. S. E. P. changes were measured preirradiation (control), immediately after laser irradiation and 4 hours postirradiation to evaluate whether each S. E. P. change immediately after laser irradiation was able to recover (S. E. P. recover) or not (S. E. P. non-recover). After S. E. P. studies, irradiated spinal cord was removed for histological examination.
The group of S. E. P. changes showing the disappearance of the early components immediately after laser irradiation revealed non-recover 4 hours post-irradiation in S. E. P. waveform. This group revealed necrosis at the site of irradiation on histological examination. There were two groups, recover one and non-recover one, in the group of S. E. P. changes showing the amplitude reduction of the early components immediately after laser irradiation. The non-recover group revealed necrosis and the recover group revealed edema in the irradiated dorsal column. The initial S. E. P waveform and its temporal change after laser irradiation could suggest the degree of tissue alteration in the irradiated lesion.
Between S.E.P. recover group and S. E. P. non-recover group, there was a significant difference (P<0.01) in percent value of the amplitude reduction of the early components of S.E.P. immediately after laser irradiation in comparison with each control. The results in this experiments suggest that it is important to monitor S. E. P. during laser surgery in the spinal cord.

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