Intrathoracic petechiae are characteristic of the postmortem changes found in sudden infant death syndrome. The presence and distribution of these petechiae have been claimed as evidence for airway obstruction as the mechanism of death. In a group of young, mature rats that were free of infection, hypoxic asphyxia produced an insignificant number of petechiae, whereas in all littermates infected with an enzootic virus (Sendai) large numbers of petechiae with hypoxic asphyxia developed. Rats similarly infected, but killed with an overdose of pentobarbital sodium, had no petechiae. Most importantly, infected rats with unremitting airway obstruction were free of petechiae. Thus, the experimental conditions necessary for the presence of intrathoracic petechiae are profound hypoxia and infection, with persistent circulation and respiratory effort; persistent airway obstruction does not produce petechiae, with or without infection.