The ammonia compensation point ( ) controls the direction and magnitude of NH3 exchange between plant leaves and the atmosphere. Very limited information is currently available on how responds to anticipated climate changes. Young barley plants were grown for 2 weeks at ambient (400 μmol mol(-1)) or elevated (800 μmol mol(-1)) CO2 concentration with or NH4NO3 as the nitrogen source. The concentrations of and H(+) in the leaf apoplastic solution were measured along with different foliar N pools and enzymes involved in N metabolism. Elevated CO2 caused a threefold decrease in the concentration in the apoplastic solution and slightly acidified it. This resulted in a decline of the from 2.25 and 2.95 nmol mol(-1) under ambient CO2 to 0.37 and 0.89 nmol mol(-1) at elevated CO2 in the and NH4NO3 treatments, respectively. The decrease in at elevated CO2 reflected a lower N concentration (-25%) in the shoot dry matter. The activity of nitrate reductase also declined (-45 to -60%), while that of glutamine synthetase was unaffected by elevated CO2. It is concluded that elevated CO2 increases the likelihood of plants being a sink for atmospheric NH3 and reduces episodes of NH3 emission from plants.