Carbon monoxide (CO) causes cellular hypoxia and cytotoxicity and functions as a neurotransmitter in the brain by binding to multiple heme proteins. Suliman and colleagues (p. 299) now demonstrate a new physiological role for CO in cardiac mitochondrial biogenesis. Mitochondrial biogenesis is regulated by coordinate expression of specialised transcriptional coactivators and factors, which increases expression of mitochondrial proteins, mitochondrial density and mitochondrial DNA copy number. The authors show that all of this occurs in mouse cardiac tissue following exposure to CO. The effect is independent of both nitric oxide synthase (NOS) and hypoxia, which indicates that it is not a general effect of endogenously formed gases. CO activates two key enzymes involved in mitochondrial biogenesis: guanylate cyclase and PKB. Nitric oxide (NO) also activates the guanylate cyclase pathway whereas PKB is only activated by CO. The authors demonstrate that PKB activation in the heart and cardiomyocytes requires increased mitochondrial H2O2 production and release, as a result of CO binding to mitochondrial cytochrome a3. Their study defines both CO and H2O2 as novel activating factors in cardiac mitochondrial biogenesis.