Exposure of oilseed rape (Brassica napus) plants to increasing leaf temperatures between 15°C and 25°C increased photorespiratory NH4+ production from 0.7 to 3.5 µmol m-2 s-1. Despite the 5-fold increase in the rate of NH4+ production, the NH4+ concentration in root and leaf tissue water and xylem sap dropped significantly, whereas that in the leaf apoplastic fluid remained constant. The in vitro activity of glutamine synthetase (GS) in both leaves and roots also increased with temperature and in all cases substantially exceeded the observed rates of photorespiratory NH4+ production. The surplus of GS in oilseed rape plants was confirmed using GS2 antisense plants with 50% to 75% lower in vitro leaf GS activity than in the wild type. Despite the substantial reduction in GS activity, there was no tendency for antisense plants to have higher tissue NH4+ concentrations than wild-type plants and no overall correlation between GS activity and tissue NH4+ concentration was observed. Antisense plants exposed to leaf temperatures increasing from 14°C to 27°C or to a trifold increase in the O2 to CO2 ratio did not show any change in steady-state leaf tissue NH4+ concentration or in NH3 emission to the atmosphere. The antisense plants also had similar leaf tissue concentrations of glutamine, glycine, and serine as the wild type, whereas glutamate increased by 38%. It is concluded that photorespiration does not control tissue or apoplastic levels of NH4+ in oilseed rape leaves and, as a consequence, that photorespiration does not exert a direct control on leaf atmosphere NH3 fluxes.