Disk herniation is a painful pathological condition in which nucleus pulposus (NP) leaks from the disk and induces inflammation. It is not well understood how the leakage of NP induces inflammation, though some mediators are implicated in NP-induced inflammation. Prostaglandins (PGs), lipid mediators involved in inflammatory pain, are possibly involved in NP-induced inflammation. The purpose of this study is to examine the expression of enzymes responsible for PG biosynthesis in intact NP of mice. NP of adult male mice was processed for immunohistochemistry and/or western blot analysis. The immunohistochemical study revealed that cyclooxygenase-1 (COX-1)-like immunoreactivity is highly expressed in NP. The expression level was the highest among other tissues examined including the brain, spinal cord, lungs, stomach, kidneys and intestines. The western blot analysis showed a single band of approximately 72 kDa, the expected molecular weight of COX-1. No COX-1-like immunoreactivity was detected in NP of COX-1-deficient mice. Microsomal prostaglandin E synthase-1 (mPGES-1), the terminal enzyme for PGE2 synthesis, and cytosolic phospholipase A2 were also constitutively expressed in NP. These results suggest that NP cells, when leaks out, could be the source of PGE2, which induces inflammatory pain in disk herniation.