Although essentially unknown, several functions are hypothesized for neuroglobin and cytoglobin, two new members of the globin family. In this article, we try to shed more light on their possible roles in hypoxia and detoxification of reactive oxygen species in vivo. The relative transcriptional changes of neuroglobin and cytoglobin in a situation of chronic hypoxia in mice were examined using real-time quantitative PCR. The kinetics of the hypoxic expression of neuroglobin (brain, eyes) and cytoglobin (brain, eyes, liver, heart, skeletal muscle) is organ-specific. Moreover, reactive oxygen species production is higher in liver than in the other tissues. In eyes, the negative correlation, after reoxygenation, between neuroglobin protein level and H(2)O(2) concentration is a first proof of a reactive oxygen species-scavenging function for neuroglobin. In addition, apoptotic cell death after hypoxia is for the first time demonstrated in heart and liver.