Phosphinothricin (glufosinate), an irreversible inhibitor of glutamine synthetase, causes an inhibition of photosynthesis in C3 (Sinapis alba) and C4 (Zea mays) plants under atmospheric conditions (400 ppm CO2, 21% O2). This photosynthesis inhibition is proceeding slower in C4 leaves. Under non-photorespiratory conditions (1000 ppm CO2, 2% O2) there is no inhibition of photosynthesis. The inhibition of glutamine synthetase by phosphinothricin results in an accumulation of NH4 (+). The NH4 (+)-accumulation is lower in C4 plants than in C3 plants. The inhibition of glutamine synthetase through phosphinothricin in mustard leaves results in a decrease in glutamine, glutamate, aspartate, asparagine, serine, and glycine. In contrast to this, a considerable increase in leucine and valine following phosphinothricin treatment is measured. With the addition of either glutamine, glutamate, aspartate, glycine or serine, photosynthesis inhibition by phosphinothricin can be reduced, although the NH4 (+)-accumulation is greatly increased. This indicates that NH4 (+)-accumulation cannot be the primary cause for photosynthesis inhibition by phosphinothricin. The investigations demonstrate the inhibition of transmination of glyoxylate to glycine in photorespiration through the total lack of amino donors. This could result in a glyoxylate accumulation inhibiting ribulose-1,5-bisphosphate-carboxylase and consequently CO2-fixation.