The purpose of this study was to examine whether mRNA of interleukin-1β, interleukin-6, phospholipase A2, and nitric oxide synthase was expressed in the nerve root and dorsal root ganglion of an animal model in which exposure to the nucleus pulposus induced mechanical hyperalgesia, a pain-related behavior. Autologous nucleus pulposus obtained from coccygeal discs was relocated on the L4 and L5 lumbar nerve roots after partial laminectomy. The reflex response to noxious mechanical stimuli to the hindpaw was measured preoperatively and to 4 weeks postoperatively. With a reverse transcription-polymerase chain reaction technique, expression of interleukin-1β, interleukin-6, phospholipase A2, and inducible nitric oxide synthase genes in the nerve root and dorsal root ganglion was observed at 1, 2, and 4 weeks postoperatively. Mechanical hyperalgesia was observed from 3 days to 2 weeks postoperatively. The expression of interleukin-1β, phospholipase A2, and inducible nitric oxide synthase mRNAs increased after only 1 week. This increase was related to mechanical hyperalgesia. Expression of interleukin-6 was detected in the neural tissue over time. It is possible that interleukin-1β, phospholipase A2, and inducible nitric oxide in the nerve root or dorsal root ganglion, or in both, produce sciatic pain in the early stage of herniation of the lumbar disc.