Wheat was grown in the field with different levels of nitrogenous fertilizer, and the rate of photosynthesis and the activity of ribulose 1,5-diphosphate carboxylase in the flag leaves determined. Additional nitrogen increased the dry-weight and leaf area of the plants, but did not increase grain yield; the rate of photosynthesis of the flag leaves was unchanged but the activity of ribulose 1,5-diphosphate carboxylase increased. The significance of these observations to the loss of potential yield of wheat and the relationship between, photosynthesis and carboxylase activity is considered.