A factorial experiment was conducted to determine the pattern of growth and grain production of wheat to intermittent exposure to SO2, and how this response is changed by the additional presence of NO2. Plants were grown in open-top chambers and exposed to one of five levels of SO2 (< 5 (control), 55, 149, 262 or 544 nl 1-1), and to one of two levels of NO2 (< 5 (control) or 170 nl 1-1) for 4 h per day for 108 days to grain harvest. SO2 alone had little effect at concentrations up to 149 nl 1-1 but at concentrations of greater than 262 nl 1-1, growth was very severely affected. Incorporation of NO2 significantly influenced both the vegetative growth and grain yield of plants. While the increase in the mean plant dry matter production in the presence of NO2 was only 47%, the corresponding increase in the mean grain yield was 118%. The NO2-induced increases in the number and weight of grain occurred with an increased vegetative growth. These changes in plant production cannot be explained as simple responses to fertilization, as plant sulphur and nitrogen concentrations were adequate. These effects, and the increase in the amount of protein stored in grain are consistent with re-allocation of resources from vegetative growth to maturing grain. Thus, in addition to having the normal fertilizer effect, NO2 also has influenced the partitioning of photoassimilates, favouring the production of grain. In the absence of NO2, the fraction of the biological yield which was allocated towards grain filling was 0.375 (S.E. = 0.015) and when NO2 was added this significantly (*P < 0.001) increased to 0.515 (S.E. = 0.011).