The purposes of this study were to detect the indices of evaluating for degree of compression and to detect the indices of recovery after decompression in different time intervals in the animal model of 50 percent compression of the cauda equina, through the examination of neurologic status and electrodiagnosis. Twenty-one male dogs, weighing between nine and eleven kilogram, were used. Three experimental groups were studied. One group (5 dogs) served as controls. In the other groups, the cauda equina was constricted by 50 percent to produce chronic compression, and decompression was done after 2 weeks compression in one group and 4 weeks compression in another group. The dogs in which the cauda equina had been decompressed at 2 weeks, showed the recovery of sensory evoked potentials at 2 weeks after decompression and bulbocavernosus reflexes at 4 weeks. The dogs in which the cauda equina had been decompressed at 4 weeks, showed the recovery of sensory evoked potentials and bulbocavernosus reflexes at 4 weeks after decompression. In both groups of the cauda equina compression, motor evoked potentials were partially recovered by 6 weeks after decompression. The initial neurological deficits in both groups were improved by 6 weeks. The cauda equina revealed that the epidural scar tissue in compressed site was more severe in 4 weeks compression group. But the substance of the cauda equina did not have any pathological changes in gross examination. Microscopically, neural tissue was showed the normal appearance without pathological changes in all groups. In conclusions, the early decompression of cauda equina revealed that the onset of recovery was more faster and the amplitude of recovery was larger comparing with delayed decompression in electrodiagnostic test. Also, the surgical decompression itself was helpful to improve the electrodiagnostic changes around 4 weeks after decompression. Sensory evoked potential was improved at the first and motor evoked potentials was recovered after disappearance of the claudication.