The increase in atmospheric CO ₂ concentration [CO ₂] has been demonstrated to stimulate growth of C ₃ crops. Although barley is one of the important cereals of the world, little information exists about the effect of elevated [CO ₂] on grain yield of this crop, and realistic data from field experiments are lacking. Therefore, winter barley was grown within a crop rotation over two rotation cycles (2000 and 2003) at present and elevated [CO ₂](375 ppm and 550 ppm) and at two levels of nitrogen supply (adequate (N2): 262 kg ha ^-1 in 1st year and 179 kg ha ^-1 in 2nd year) and 50% of adequate (N1)). The experiments were carried out in a free air CO ₂ enrichment (FACE) system in Braunschweig, Germany. The reduction in nitrogen supply decreased seasonal radiation absorption of the green canopy under ambient [CO ₂] by 23%, while CO ₂ enrichment had a positive effect under low nitrogen (+8%). Radiation use efficiency was increased by CO ₂ elevation under both N levels (+12%). The CO ₂ effect on final above ground biomass was similar for both nitrogen treatments (N1: +16%; N2: +13%). CO ₂ enrichment did not affect leaf biomass, but increased ear and stem biomass. In addition, final stem dry weight was higher under low (+27%) than under high nitrogen (+13%). Similar findings were obtained for the amount of stem reserves available during grain filling. Relative CO ₂ response of grain yield was independent of nitrogen supply (N1: +13%; N2: +12%). The positive CO ₂ effect on grain yield was primarily due to a higher grain number, while changes of individual grain weight were small. This corresponds to the findings that under low nitrogen grain growth was unaffected by CO ₂ and that under adequate nitrogen the positive effect on grain filling rate was counterbalanced by shortening of grain filling duration.