Triadimefon (TDM), a systemic fungicide with non-traditional plant-growth regulator properties, was administered to Catharanthus roseus (L.) G. Don. plants in order to determine its effects on oxidative injury in terms of H₂O₂ content, lipid peroxidation (LPO), electrolyte leakage (EL), protein and amino acid contents, as well as proline metabolism. The LPO, estimated as thiobarbituric acid-reactive substances (TBARS), decreased under TDM treatment. It was found that H₂O₂ and EL were reduced under TDM treatment when compared to control. TDM treatment caused a significant increase in the protein and amino acid contents. Glycine betaine (GB) and proline (PRO) significantly accumulated in C. roseus under stress arisen from fungicide applications. Proline oxidase (PROX) activities reduce the PRO content and γ-glutamyl kinase (γ-GK) accelerates the synthesis of PRO. Under TDM treatment, the activity of PROX decreased and the γ-GK activity increased. From our results, it is suggested that fungicide triadimefon causes activation of metabolic processes in the medicinal plant Catharanthus roseus. These findings are of great significance for the cultivation of this medicinal plant, as it was previously reported that TDM causes an enhancement of antioxidant metabolism and ajmalicine production in C. roseus. To cite this article: C.A. Jaleel et al., C. R. Biologies 330 (2007).