Diurnal fluctuations in dry matter accumulation and leaf extension of seedlings of Dactylis glomerata were followed through a 16-h light period and a subsequent 8-h dark period at 20 °C.The measured increase in dry weight during the light period and the decrease during the dark period showed a very good agreement with calculated dry weight changes derived from the rates of carbon dioxide exchange of whole seedlings. Although dry weight of the leaf blades decreased during the dark period, leaf expansion continued throughout the 24-h period with associated changes in the ratio of fresh weight to dry weight of the leaf blades.