Various kinds of chemical mediators are synthesized in the course of pulpitis; thus, control of their production would assist in inducing a reduction in pulpal inflammation. We hypothesized that nitric oxide (NO) would be an important mediator of pulpal inflammation. Pulpal inflammation was induced by the application of LPS in rat incisor pulp, and inducible nitric oxide synthase (iNOS) expression was evaluated by reverse-transcription/polymerase chain-reaction and immunohistochemical staining. After LPS application, iNOS mRNA was first detected after 3 hrs, peaked at 6 hrs, and decreased thereafter. iNOS-positive cells were macrophages and neutrophils. An NOS inhibitor caused drastic decreases in the expression of pro-inflammatory cytokines and COX2 mRNA, which was highly induced in the LPS-induced pulpitis. These results indicate that NO synthesis is related to the initiation of mediator production, and that its down-regulation should contribute to the prevention of pro-inflammatory mediator synthesis.