The imbalance between high oxidant loads and immature antioxidant defenses is associated with long-term complications of prematurity. Glutathione is a central element among the antioxidants. Depletion of pulmonary glutathione accelerates the development of oxygen-induced lung injury in neonatal animal models. After the observation that newborn infants exposed to oxygen have low glutathione levels, a study was designed to test the hypothesis that in neonates from a species susceptible to oxygen toxicity, the lethal effect of hyperoxia is related to a low availability of substrates for glutathione production rather than an impairment in synthetic activity. One-day-old guinea pigs, randomly assigned to room air or oxygen (>95%), were fed by their mothers (n = 16) or i.v. by dextrose (n = 14) or by total parenteral nutrition (TPN, n = 20). After 3 d, glutathione and activities of enzymes involved in maintaining intracellular glutathione levels were determined in lungs and liver. The lethal effect of oxygen (p < 0.05) observed in animals without TPN was not related to glutathione depletion, as oxygen induced a 33% increase in lung glutathione, positively correlated (r2 = 0.35) with enhanced synthesis. With TPN, the animals were protected against the lethal effects of hyperoxia and lung glutathione increased by 67% in oxygen. The results suggest that the glutathione demand by the lungs in the presence of an oxidant stimulus was met by the increased (p < 0.001) hepatic production supported by TPN. Under hyperoxic conditions, early nutritional support is of vital importance.