Pancreatic islets transplanted to treat autoimmune type 1 diabetes often fail to function (primary nonfunction), likely because of islet beta-cell apoptosis. We show that carbon monoxide (CO), a product of heme oxygenase activity, protects beta-cells from apoptosis. Protection is mediated through guanylate cyclase activation, generation of cyclic GMP (cGMP), and activation of cGMP-dependent protein kinases. This antiapoptotic effect is still observed when beta-cells are exposed to CO for 1 h before the apoptotic stimulus. In a similar manner, mouse islets exposed to CO for just 2 h function significantly better after transplantation than islets not exposed to CO. These findings suggest a potential therapeutic application for CO in improving islet function/survival after transplantation in humans.