The aim of this work was to establish the quantitative impact of photorespiration on leaf amino acid contents. Attached leaves of wheat and potato were incubated for 30–40 min under defined conditions in which net CO2 uptake (A) was manipulated by irradiance, ambient CO2 or ambient O2. The incubated portion of the leaf was sampled by a rapid-quench method and photorespiratory flux (vo) was modelled from the measured rate of net CO2 uptake. In both wheat and potato, the ratio between glycine and serine showed a strong positive correlation with vo. Aspartate and alanine correlated negatively with vo but glutamate and glutamine showed less clear relationships. In potato, glutamate and glutamine did not correlate clearly with either A or vo. In wheat, glutamine showed a general increase with A but no relationship with vo, whereas 2-oxoglutarate contents correlated positively with vo and negatively with A. As a result, glutamine : glutamate and glutamine : 2-oxoglutarate increased with net CO2 uptake in wheat, observations that are attributed primarily to imperfect and variable coupling between the supply of NH3 in primary nitrogen assimilation and the associated delivery of 2-oxoglutarate to the chloroplast. A simple theoretical analysis is used to illustrate the potentially marked impact of primary nitrogen assimilation on leaf glutamine, even against a background of high rates of photorespiratory ammonia recycling.