Seedlings of Helianthus annuus L. were grown at an initially high relative nitrate supply rate (0.27 mol N mol N–1 d–1). The supply was subsequently reduced to a low rate (0.04 mol N mol N–1 d–1). The response of leaf area development to this abrupt decrease in nitrate availability was characterized by following the expansion of the primary and secondary leaf pairs. The timing of the drop in nitrate supply was when cell division in the epidermis of the primary leaf pair was largely complete. Reducing the availability of nitrate had a strong effect on leaf area expansion. The final leaf size of the primary leaf pair was affected indicating an effect of nitrate availability on cell expansion. By the end of the experiment the secondary leaf pair was only one-third the area of that on control seedlings. The role of epidermal cell turgor pressure in this growth response was assessed by direct measurements with a miniature cell pressure probe. No reduction in cell turgor pressure following the decrease in nitrate availability was detected. It is concluded that a reduction in turgor pressure was not responsible for the reduction in leaf area expansion and it is suggested that reduced cell expansion was due to changes in cell wall properties. Concentrations of leaf and root abscisic acid increased following the reduction in nitrate availability.