The regulation of Rubisco in terms of activation and the presence of inhibitors was assessed in the leaves of wheat plants, in response to drought and heat stress. A decline in the leaf relative water content by 25% caused little change in the initial and total Rubisco activity or activation state. There was, however, some evidence for a decreased amount of Rubisco protein in drought-stressed plants, shown by reduced maximal Rubisco activity and CABP binding. In the droughted plants, there was a significant reduction in the concentration of the ‘daytime inhibitor’ of Rubisco activity, indicating that it did not play an important role during drought stress in this species. High temperature stress caused a reduction in photosynthetic rate. As the temperature was increased from 25 to 40 °C, the photosynthetic rate was reduced by 100%, followed by a 90% reduction in stomatal conductance, although the internal CO2 content increased slightly. The initial and total Rubisco activity and activation state declined with increasing temperature, suggesting a possible perturbation of the heat-labile Rubisco activase activity. The amount of RuBP declined as temperature increased, suggesting a reduction in the flux through the Calvin cycle. The decrease in concentration of the ‘daytime inhibitor’ of Rubisco activity indicated that, as for drought, it did not play an important role during temperature stress in this species.