In order to analyze the signaling function of hydrogen peroxide (H2O2) production in senescence in more detail, we manipulated intracellular H2O2 levels in Arabidopsis thaliala (L.) Heynh by using the hydrogen-peroxide-sensitive part of the Escherichia coli transcription regulator OxyR, which was directed to the cytoplasm as well as into the peroxisomes. H2O2 levels were lowered and senescence was delayed in both transgenic lines, but OxyR was found to be more effective in the cytoplasm. To transfer this knowledge to crop plants, we analyzed oilseed rape plants Brassica napus L. cv. Mozart for H2O2 and its scavenging enzymes catalase (CAT) and ascorbate peroxidase (APX) during leaf and plant development. H2O2 levels were found to increase during bolting and flowering time, but no increase could be observed in the very late stages of senescence. With increasing H2O2 levels, CAT and APX activities declined, so it is likely that similar mechanisms are used in oilseed rape and Arabidopsis to control H2O2 levels. Under elevated CO2 conditions, oilseed rape senescence was accelerated and coincided with an earlier increase in H2O2 levels, indicating that H2O2 may be one of the signals to inducing senescence in a broader range of Brassicaceae.