The effects of elevated concentrations of atmospheric carbon dioxide and ozone on diurnal patterns of photosynthesis have been investigated in field-grown spring wheat (Triticum aestivum). Plants cultivated under realistic agronomic conditions, in open-top chambers, were exposed from emergence to harvest to reciprocal combinations of two carbon dioxide and two ozone treatments: [CO2] at ambient (380 μmol mol−1, seasonal mean) or elevated (692 μmol mol−1) levels, [O3] at ambient (27 nmol mol−1, 7 hr seasonal mean) or elevated (61 nmol mol−1) levels. After anthesis, diurnal measurements were made of flag-leaf gas-exchange and in vitro Rubisco activity and content. Elevated [CO2] resulted in an increase in photoassimilation rate and a loss of excess Rubisco activity. Elevated [O3] caused a loss of Rubisco and a decline in photoassimilation rate late in flag-leaf development. Elevated [CO2] ameliorated O3 damage. The mechanisms of amelioration included a protective stomatal restriction of O3 flux to the mesophyll, and a compensatory effect of increased substrate on photoassimilation and photosynthetic control. However, the degree of protection and compensation appeared to be affected by the natural seasonal and diurnal variations in light, temperature and water status.