Aim: To observe the effect of nucleus pulposus allografted into the epidural space on pain threshold in rats, so as to provide theoretical evidence for discogenic pain. Methods: The experiment was carried out i n the animal experimental center of Xuanwu Hospital, Capital University of Medical Sciences between June and August 2005. Twenty-seven male Sprague-Dawley rats were randomly divided into three groups with 6 rats in each: sham-operated group, fat grout and nucleus pulposus group. The allografted fat and nucleus pulposus were provided by the other 9 male SD rats. Rats in the sham-operated group were given an epidural space exposure without any graft implantation. In the fat group and nucleus pulposus group, rats had allografts of adipose tissue (approximately 10 mg) and nucleus pulposus (approximately 3.0-4.0 mg) implanted into epidural space, respectively. Preoperatively and at 3, 7, 15 and 30 days postoperatively, the responses of tail to transcutaneous electrostimulation (to observe the current caused by tail flick, which was the electrostimulated tail-flick threshold value), thermal stimulation (to observe the duration from immersion to tail flick, which was the tail flick latency to thermal stimulation) and mechanical stimulation (to observe the total mass of counterpoises when tail flick, which was the tail flick threshold value to mechanical stimulation) were observed. Results: All the 18 rats were involved in the analysis of results. The postoperative threshold values of rats to transcutanenous electrostimulation, thermal stimulation and and mechanical stimulation in the sham-operated group and fat group were not significantly different from the preoperative ones (P > 0.05). In the nucleus pulposus group, there were electrical, mechanical and thermal hyperalgesias after nucleus pulposus allografted into the epidural space. At 15 Days postoperatively, the changed percentages of tail flick to electrostimulation in the sham-operated group, fat group and nucleus pulposus group were 1.08%, 4.74% and -53.42%, respectively (P < 0.05), those to mechanical stimulation were 2.10%,-1.21% and -43.77% respectively (P < 0.05), and those to thermal stimulation were -4.12%, 1.07% and -16.43% respectively (P < 0.05). Conclusion: Nucleus pulposus allografted into the epidural space can lead to the hyperalgesia of rats, which may be one of the reasons for radicular pain of a lumbar disc herniation results from nucleus polposus.