To investigate how nitrogen supply might affect the biophysical factors controlling diurnal variation in leaf extension, pot-grown Salix viminalis L. were supplied with nitrogen at a low relative addition rate of 0.05 g N g(-1) N day(-1) (low N) or were given free access to all nutrients (high N). Leaf extension, turgor pressure, turgor after stress relaxation and the plastic extensibility of leaf tissue were determined for growing leaves every 4 h during two days of clear skies in August. Plants in the high-N treatment had a significantly higher relative growth rate, dry weight, shoot/root ratio, leaf nitrogen concentration, total leaf area, final area of single leaves and epidermal cell size than plants in the low-N treatment. The periodicity of leaf extension was similar in both treatments with high values during the afternoon and early evening, and negligible values during the night and in the early morning. The maximum rate of leaf extension was higher in high-N than in low-N plants. Leaf water potential and leaf osmotic potential decreased in the morning and increased in the afternoon with highest values during the night. Calculated values of turgor pressure showed no consistent diurnal trend and did not correlate with the rate of leaf extension. There was no consistent difference in turgor between treatments. Turgor after stress relaxation varied diurnally. The difference between turgors before and after stress relaxation also varied diurnally and was largely in phase with the diurnal pattern of leaf extension. These data are consistent with either a causal role for growth turgor (difference between turgors before and after stress relaxation) in the regulation of cell expansion, or a diurnal variation in turgors after relaxation, attributable to different capacities for cell wall loosening at different times of day. Plastic extensibility of leaf tissue showed no diurnal pattern but consistently higher values were found in high-N than in low-N plants. We conclude that the effects of nitrogen supply on leaf water relations did not limit leaf extension, but that nitrogen supply did affect processes associated with cell wall loosening and enlargement. Nitrogen supply did not affect final values of turgor after relaxation, but it presumably affected the rate at which relaxation proceeded.