Peroxisome proliferator-activated receptors (PPAR) decrease the production of cytokine and inducible nitric-oxide synthase (iNOS) expression, which are associated with aging-related inflammation and insulin resistance. Recently, the involvement of the induction of heme oxygenase-1 (HO-1) in regulating inflammation has been suggested, but the exact mechanisms for reducing inflammation by HO-1 remains unclear. We found that overexpression of HO-1 and [Ru(CO)3Cl2]2, a carbon monoxide (CO)-releasing compound, increased not only ERK5 kinase activity, but also its transcriptional activity measured by luciferase assay with the transfection of the Gal4-ERK5 reporter gene. This transcriptional activity is required for coactivation of PPARδ by ERK5 in C2C12 cells. [Ru(CO)3Cl2] 2 activated PPARδ transcriptional activity via the MEK5/ERK5 signaling pathway. The inhibition of NF-κB activity by ERK5 activation was reversed by a dominant negative form of PPARδ suggesting that ERK5/PPARδ activation is required for the anti-inflammatory effects of CO and HO-1. Based on these data, we propose a new mechanism by which CO and HO-1 mediate anti-inflammatory effects via activating ERK5/PPARδ, and ERK5 mediates CO and HO-1-induced PPARδ activation via its interaction with PPARδ.