Renal ischemia/reperfusion injury is a major complication of kidney transplantation. We tested if ex vivo delivery of carbon monoxide (CO) to the kidney would ameliorate the renal injury of cold storage that can complicate renal transplantation. Orthotopic syngeneic kidney transplantation was performed in Lewis rats following 24 h of cold preservation in University of Wisconsin solution equilibrated without or with CO (soluble CO levels about 40 microM). Ischemia/reperfusion injury in control grafts resulted in an early upregulation of inflammatory mediator mRNAs and progressive deterioration of graft function. In contrast, the grafts preserved with CO had significantly less oxidative injury and this was associated with improved recipient survival compared to the control group. Renal injury in the control group showed considerable degradation of cytochrome P450 heme proteins, active heme metabolism and increased detrimental intracellular free heme levels. Kidney grafts preserved in CO-equilibrated solution maintained their cytochrome P450 protein levels, had normal intracellular heme levels and had less lipid peroxidation. Our results show that CO-mediated suppression of injurious heme-derived redox reactions offers protection of kidney grafts from cold ischemia/reperfusion injury.