As the leaf area expansion and the foliar regression are governed by the sunflower water and N status, the interactions of these factors on leaf area index (LAI) and duration (LAD), and their relationships with yield components, are studied in a field experiment involving different soil depths, N fertilizations and irrigations and two hybrid cultivars. The plants were allowed to take up 100 to 470 kg-ha-1 N, and 200 to 700 mm of water; they were free of any diseases or lodging. Twenty-five typical plants per plot were tagged at the 12 leaves stage, and some of them were sampled at the stages: bud apparition, beginning and end of anthesis, and maturity, for measurements of dry matter and N content in leaves, stems and heads. Yields vary from about 1.5 to 4.5 tons -ha-1; N supply mainly affects the number of achenes per head and their protein contentl water has a strong effect on this number, on the average weight of achenes and on their oil percenlage. The LAD is chiefly dependant on the water regimen, with some modulation according to N availabilities. A LAI of about 2.5 is sufficient for the maximum growth of the shorlest hybrid, the taller one requiring roughly 4. From the stage of bud enlargcment, the numbcr of achenes is strongly correlated both with LAI and ttte N content in leaves and stems. The average weight of achenes is well-correlated with LAI beyond anthesis and then with the available water. N mainly determines the potential number of achenes at eârly stages, and wâter their setting and filling' with a predominant effect on LAI and LAD.