The response of underground plant tissues to O-2 limitation is currently an important topic in crop plants since adverse environmental conditions (e.g. waterlogging) may cause root hypoxia and thus compromise plant growth. However, little is known on the effect of low O-2 conditions in leaves, probably because O-2 limitation is improbable in these tissues under natural conditions, unless under complete submersion. Nevertheless, an O-2-depleted atmosphere is commonly used in gas exchange experiments to suppress photorespiration and estimate gross photosynthesis. However, the nonphotosynthetic effects of gaseous O, depletion, particularly on respiratory metabolism, are not well documented. liere, we used metabolomics obtained under contrasting O, and CO, conditions to examine the specific effect of a changing O-2 mole fraction from ambient (21%) to 0%, 2% or 100%. In addition to the typical decrease in photorespiratory intermediates (glycolate, glycine and serine) and a build-up in photosynthates (sucrose), low O, (0% or 2%) was found to trigger an accumulation of alanine and change succinate metabolism. In 100% O-2, the synthesis of threonine and methionine from aspartate appeared to be stimulated. These responses were observed in two species, sunflower (Helianthus annuus L.) and Arahidopsis thaliana (L.) Heynh. Our results show that O-2 causes a change in the oxygenation : carboxylation ratio and also alters postphotosynthetic metabolism: (i) a hypoxic response at low O, mole fractions and (ii) a stimulation of S metabolism at high O, mole fractions. The latter effect is an important piece of information to better understand how photorespiration may control S assimilation.