The net assimilation rate of wheat, on a leaf area basis, declined with time under constant environmental conditions. With continuous illumination at temperatures of 20, 25, and 30 °C, the net assimilation rate increased linearly with increases in the light intensity over the range 200 to 5000 ft-c. The optimal temperature was 15 to 20 °C, and the net assimilation rate declined with increases in temperature over the range 20 to 30 °C. These responses are related to direct effects of light intensity on apparent rates of leaf photosynthesis and to changes in the relative proportion of photosynthetic and primarily respiratory organs under different environmental conditions.