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To determine if some specific “preparation for birth” occurs in the developing lung to help assure its successful adaptation to a comparatively O2-rich world at birth, we measured the activities of the antioxidant enzymes in the developing lungs of rabbit fetuses from 10 d before parturition to several days after birth. Superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GP) activities showed similar maturational patterns with significant increases in activity, compared with earlier gestational levels, during the last 3-5 d before birth. During the final days in utero, SOD and CAT activities increased by ~110 and lung GP activity by ~200. There were no parallel changes in lung O2consumption demonstrable over this same prenatal period.