Spring wheat (Triticum aestivum L. cv. Albis) was grown during two consecutive seasons (1987, 1988) in open-top field fumigation chambers at a site in central Switzerland and exposed to different doses of ozone (O3) in order to study O3 effects on growth and leaf senescence. The O3 dose in nonfiltered air (NF) was 42.6 μl I-1 h in 1987 and 49.2 μ I-1 h in 1988. The other treatments used were: charcoal-filtered air (CF) (0.5 × O3 dose in nonfiltered air) and nonfiltered air with one of two levels of O3 added daily (0900–1700 h) (O3-1 with 1.5 × O3 dose in nonfiltered air and O3-2 with 1.9 × O3) dose in nonfiltered air). From anthesis through senescence, dry weight of individual ears, and chlorophyll and protein content, glutamate dehydrogenase activity (GDH, EC 184.108.40.206), and the content of free and conjugated 1-aminocyclopropane-1-carboxylic acid (ACC and MACC) in flag leaves were determined. With increasing O3 dose, the decline in chlorophyll and soluble protein contents started earlier, but the rate of decline decreased. Independent of the treatment used, GDH activity was highest during the period of maximal protein loss. With increasing O3 dose, the content of MACC increased, while that of free ACC remained unaffected. In 1988, temporal changes in MACC were related to other senescence processes. It is suggested that with increasing stress from chronic O3 exposure leaf duration is reduced, but that the pattern of senescence processes remains unchanged. Ozone-induced acceleration of flag leaf senescence could contribute to the observed reduction in ear dry weight accumulation, which may account for yield reduction in spring wheat.