IL-10 is a potent anti-inflammatory and immune regulatory cytokine. IL-10−/− mice produce exaggerated amounts of inflammatory cytokines when stimulated with LPS, indicating that endogenous IL-10 is a central regulator of inflammatory cytokine production in vivo. PGs are lipid mediators that are also produced in large amounts during the inflammatory response. To study the role of IL-10 in the regulation of PG production during the acute inflammatory response, we evaluated LPS-induced cyclooxygenase (COX) expression and PG production in wild-type (wt) and IL-10−/− mice. LPS-induced PGE2 production from IL-10−/− spleen cells was 5.6-fold greater than that from wt spleen cells. LPS stimulation resulted in the induction of COX-2 mRNA and protein in both wt and IL-10−/− spleen cells; however, the magnitude of increase in COX-2 mRNA was 5.5-fold greater in IL-10−/− mice as compared with wt mice. COX-1 protein levels were not affected by LPS stimulation in either wt or IL-10−/− mice. Neutralization of IFN-γ, TNF-α, or IL-12 markedly decreased the induction of COX-2 in IL-10−/− spleen cells, suggesting that increased inflammatory cytokine production mediates much of the COX-2 induction in IL-10−/− mice. Treatment of IL-10−/− mice with low doses of LPS resulted in a marked induction of COX-2 mRNA in the spleen, whereas wt mice had minimal expression of COX-2 mRNA. These findings indicate that, in addition to IL-10’s central role in the regulation of inflammatory cytokines, endogenous IL-10 is an important regulator of PG production in the response to LPS.