The TET enzymes convert methylcytosine to the newly discovered base hydroxymethylcytosine. While recent reports suggest that TETs may play a role in response to oxidative stress, this role remains uncertain and results lackin vivomodels. Here we show a global decrease of hydroxymethylcytosine in cells treated with buthionine sulfoximine and in mice depleted for the major antioxidant enzymesGPx1 and 2. Furthermore, genome-wide profiling revealed differentially hydroxymethylated regions in coding genes and intriguingly in microRNA genes, both involved in response to oxidative stress. These results thus suggest a profound effect ofin vivooxidative stress on the global hydroxymethylome.