The effects of carbon monoxide, one of the most commonly encountered toxic agents in forensic practice, have been known for a long time, but the nature of its bonding to the heme prosthetic group of hemoproteins has only recently been elucidated. In addition to reducing the oxygen capacity of the blood and the consequent systemic hypoxia, carbon monoxide interferes with the dissociation of oxyhemoglobin and the removal of carbon dioxide. Carbon monoxide also produces a direct cytotoxic effect by inactivating some intracellular respiratory enzymes, but the relative importance of these various mechanisms often remains uncertain, particularly with regard to the lesions in the brain and myocardium.