Summary The regulation of the synthesis and the levels of GSH and other acid-soluble thiols was studied in maize roots by applying GSH, GSH monoethylester, OTC (L-2-oxothiazolidine-4-carboxylic acid), the GSH precursors cysteine and γ-EC (γ-glutamylcysteine), the γ-EC synthetase (EC 6.3.2.2) inhibitor BSO (buthionine S,R-sulfoximine), and the herbicide safener benoxacor (4-dichloroacetyl-3,4-dihydro-3methyl- 2H-1,4-benzooxazine). OTC, GSH and GSH monoethylester did not affect the GSH content, whereas exogenous cysteine and γ-EC augmented it 1.6- and 2.5-fold, respectively. Both cysteine and γ-EC strongly increased their endogenous level. Benoxacor increased the activity of adenosine 5' -phosphosulfate sulfotransferase and γ-EC synthetase within 2 and 4 h, respectively, but did not affecte GSH synthetase activity (EC 6.3.2.3). It rapidly induced a high level of γ-EC and doubled the cysteine and the GSH contents. When benoxacor and γ-EC were applied together an additional increase in GSH was detected. BSO alone or together with benoxacor resulted in decreased GSH levels. Both substances combined caused an accumulation of γ-EC. These results are discussed on the basis of the localization of the GSH synthesizing enzymes in the proplastids and the cytoplasm and the intracellular site of action of benoxacor and BSO.