The REOX trial is a national multi-center double-blinded randomized clinical trial for studying the effects of resuscitation of premature infants of extremely low gestational age initiated with a low oxygen concentration in comparison to resuscitation initiated with a high oxygen level. The objectives of this clinical trial are to increase survival rate and reduce the occurrence of significant morbidities typically observed in premature infants, such as e.g. severe retinopathy of prematurity (ROP) and bronchopulmonary dysplasia (BPD). Furthermore measures of oxidative stress are expected to decrease when initiating resuscitation with low oxygen levels. Recently, several parameters measured in blood or urine samples have shown their usefulness for assessing oxidative stress. In this clinical trial in the first place the glutathione/glutathione disulfide (GSH/GSSG) ratio is determined, which is considered being a good index of the redox status representing the true effect of fetal to neonatal transition. Additional parameters for assessing protein and DNA oxidation are determined employing reliable and straightforward methods based on liquid chromatography with tandem mass spectrometric (LC-MS/MS) detection, that have been developed in our group.