The removal of Mg²⁺ is an important step in the chlorophyll degradation pathway and extracts from senescent and presenescent <i>Arabidopsis thaliana</i> leaves were analyzed for Mg-dechelatase activity, using chlorophyllin, an artificial derivative of the natural substrate, chlorophyllide. The optimum temperature and pH for this reaction were determined to be at approximately 50 °C and 7.2, respectively. Mg-dechelatase activity was enhanced by addition of EDTA and inhibited by MgCl₂, HgCl₂ and reduced glutathione, indicating phenomenons such as retroinhibition by reaction products and dependence on the redox state of the mixture. Size exclusion chromatography was performed on <i>Arabidopsis</i> leaf extracts, and Mg-dechelatase activity was found in the fraction corresponding to molecular mass of about 42 kDa, which indicates that the Mg-dechelating compound in <i>Arabidopsis</i> is considerably larger than in other systems. During dark-induced senescence, the activity increased over time until reaching a maximum at day 4, and then decreased. The addition of plant growth regulators indicated that the accumulation of Mg-dechelatase was activated by ethylene and delayed by 6-benzylaminopurine.Instituto de Fisiología Vegetal