Carbon monoxide (CO) is an odorless, tasteless and colorless gas which is generated by heme oxygenase enzymes (HOs). HOs degrade heme releasing equimolar amounts of CO, iron and biliverdin, which is subsequently reduced to bilirubin. CO shares many properties with nitric oxide (NO), an established cellular messenger. Both CO and NO are involved in neural transmission and modulation of blood vessel function, including their relaxation and inhibition of platelet aggregation. CO, like NO, binds to heme proteins, although CO binds only ferrous (FeII) heme, whereas NO binds both ferrous and ferric (FeIII). CO enhances the activity of guanylate cyclase although it is less potent than NO. In contrast, CO inhibits other heme proteins, such as catalase or cytochrome P450. The effects of CO on gene expression can be thus varied, depending on the cellular microenvironment and the metabolic pathway being influenced. In this review the regulation of gene expression by HO/CO in the cardiovascular system is discussed. Recent data, derived also from our studies, indicate that HO/CO are significant modulators of inflammatory reactions, influencing the underlying processes such as cell proliferation and production of cytokines and growth factors.