AbstractExperimentsin vitrohave shown that γ‐EC synthesis, the first step in GSH formation, is subject to feedback inhibition by physiological GSH concentrations. In order to evaluate the role of this feedback inhibition on γ‐EC synthetasein vivoGSH synthesis was modulated in suspension cultures ofP. crispumandN. tabacumby administration of cadmium. The alterations in the thiol contents were measured and in addition the effect of Cd exposure on γ‐EC synthetase (E.C. 6.3.2.2) and GSH synthetase (E.C. 6.3.2.3) was studied. Decreasing cellular GSH concentrations by cadmium induced PC synthesis caused 7–10 fold increase in the rate of glutathione synthesis as measured by the accumulation of (γ‐EC)nG. This increase was not linked to an increase in extractable activities of γ‐EC‐ or GSH synthetase in parsley. In tobacco the activities of γ‐EC‐ and GSH synthetase increased by a factor of 1.6 and 1.8, respectively, after 3 d of Cd exposure. In both species the exposure to Cd resulted in an increased cellular γ‐EC content that reached a plateau within 24 h, and in a doubling of the cysteine content.In vitroexperiments showed that GSH synthetase activity is inhibited by cadmium concentrations that have no effect on γ‐EC synthetase activity. This may explain the accumulation of γ‐EC in Cd exposed cells. Incubation with 0.25 mM cysteine did not effect the γ‐EC‐ and GSH content in tobacco cells. In parsley the cellular GSH content increased threefold and the y‐EC content twofold and stayed constant thereafter at the elevated levels. Taken together the results show that GSH synthesisin vivois controlled by feedback inhibition as well as by the supply with cysteine. In the latter case the feedback inhibition may act as a kind of safety valve and prevent the accumulation of unphysiological GSH concentrations