Grain growth was studied in six wheat lines which represented a wide range of mature grain weights (18-56 mg). Endosperm cell number, measured by counting nuclei, was positively related to final grain weight, apparently by allowing a higher rate of grain growth. Those wheat lines with fewer endosperm cells also tended to have smaller cells. As a consequence, large grains contained larger numbers of starch granules per endosperm and per cell. The time after anthesis at which nitrogen in the flag leaves began to decline varied between wheat lines and appeared to be related to both the pattern of nitrogen storage in the grain and the duration of grain growth. However, once initiated, the rate of flag-leaf senescence exhibited similar kinetics in all wheat lines.