Postillumination CO2 fixation by wheat leaves was studied following light-limited photosynthetic conditions. Dark CO2 fixation showed two phases differing by their rates of CO2 uptake and carbon metabolism. These two phases are related to preillumination light flux density. During the first 30s of darkness, assimilated CO2 was found in PGA, alanine, malate and aspartate. After 5 min of darkness, it was additionally found in phosphorylated sugars.The lack of labelling of glycolate pathway intermediates shows that the Calvin cycle cannot run in the dark.The synthesized compounds indicate that reducing power but not ATP is available after turning the light off. This observation suggests that during pre-illumination, when light strictly limits photosynthesis, ATP supply would be the first limiting factor.