Abstract: An Egyptian cultivar of wheat (Triticum aestivum L.) was exposed to 60 nl I–1 O3 (8 h d–1) in open-top chambers for 61 d. Ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC), N-malonyl 1-aminocyclopropane-1-carboxylic acid (MACC) chlorophyll contents, visible injury symptoms and yield were measured in this study. O3 was found to increase ethylene, ACC and MACC contents by 22, 29 and 38 percent, respectively, while chlorophyll content and yield were decreased by 29 and 61 per cent, respectively. The potential relationships between these parameters are discussed in the context of O3 induction of accelerated senescence. These results have significance in terms of the maintenance of agricultural yields as O3 rises in Egypt.