The postanoxic injury, also known as reperfusion injury, is associated with the returning of anoxic tissues to normal atmosphere. Using tetrazolium chloride staining, ATP content, and seedling growth rate as indicators, we found that postanoxic injuries in soybean (Glycine max) seedlings were more severe after 1 and 2 hours of anoxia than after longer anoxic durations (3 to 5 hours). Anaerobic incubation of root tips in the presence of 100 mm ascorbate, an antioxidant and free radical-scavenging compound, alleviated the postanoxic injury associated with the short durations of anoxia. Extracts from soybean seedling roots returned to air from 1 hour of anoxia had an elevated capacity to produce superoxide radicals over extracts from postanoxic roots stressed for 3 or 5 hours. Activity of superoxide dismutase in soybean roots returned to air from 1 and 2 hours of anoxia was 30 to 50% lower than activities in roots returned to air from 5 hours of anoxia. Superoxide dismutase-specific transcripts were also lower in postanoxic roots stressed for 1 hour than in roots stressed for longer anoxic durations. The evidence suggested that the postanoxic injury of soybean roots after a short anoxic stress was associated with an increased superoxide radicals production capacity coupled with a reduced superoxide dismutase activity. Periods of anoxia of at least 3 hours were necessary for soybean seedlings to develop the ability to cope with postanoxic stress.