Length, area, weight and concentrations of pigment were measured from just after emergence until senescence of the fourth leaf of Lolium temulentum L. seedlings grown in nutrient solution in a controlled environment. At the same time the progress of photosynthesis, respiration and carbon contents of the leaf were recorded. A new approach to calculating photosynthesis indices for grasses was developed, based on logistic curves fitted to leaf lengths. Dry weight and photosynthetic rates per unit area were greatest in young leaves. Subsequently photosynthetic capacity at light saturation declined, as did photosynthetic rates at the light incident on the leaf, although the decrease was less pronounced in the latter case. Dry weight per leaf remained fairly constant once full expansion was reached, with perhaps a slight rise in the oldest leaves. The photosynthesis and carbon content data were combined to calculate the carbon balance of leaf 4, with respect both to the atmosphere and to the rest of the plant. Net gas exchange became positive about 7 d before full leaf expansion. Leaf weight continued to increase so that the carbon balance with the rest of the plant did not become positive until just after final leaf size had been attained. These results are discussed in relation to the likely contribution of the various components of the balance to plant productivity.