In a prospective, randomized, blinded trial we have studied the effects of resuscitation upon oxygenation in a group of asphyxiated newly born infants receiving room air or 100% oxygen as the gas source. During the acute phase of asphyxia and until the resuscitation procedure concluded, we determined serial blood gases as well as reduced and oxidized glutathione, enzymes involved in the glutathione redox cycle, and antioxidant enzyme activities. The use of 100% oxygen caused a remarkable increase of partial pressures of oxygen in arterial blood, with values that were frequently above physiological levels (> 100 mm Hg). In addition, we have found a significant correlation between hyperoxemia and the intra-erythrocyte GSSG (oxidized glutathione) concentration. We hypothesize that hyperoxemia may be 1 of the triggering factors responsible for an increased oxidation of GSH (reduced glutathione). Moreover, an increased antioxidant enzyme activity, which reflects an oxidative stress, indicates that the antioxidant capacity of the newly born infant may have been surpassed.