Intrathoracic petechiae have been regarded as a characteristic postmortem finding in crib death, suggesting to some that upper airway obstruction was involved. Ninety rats were asphyxiated in various ways, and the frequency of petechiae was determined. Abrupt and unremitting tracheal occlusion produced petechiae in only 7%, whereas breathing 100% nitrogen produced petechiae in more than 80% of the animals. Respiratory paralysis with succinylcholine caused petechiae in only 20%, and cardiac arrest with potassium chloride in 27%. We conclude that hypoxia, prior to cardiac arrest, and vigorous respiratory movements are necessary to produce intrathoracic petechiae, and that unremitting airway obstruction is unlikely as the cause of SIDS. Instead, we propose that crib death is due to primary apnea, secondary to moderate hypoxia.