The circadian regulation of leaf hydraulic conductance (Kleaf) was investigated in Helianthus annuus L. (sunflower). Kleaf was measured with an high pressure flow meter during the light and dark period from plants growing at a photoperiod of 12 h. Kleaf was 4.0 e−4 kg s−1 m−2 MPa−1 during the light period (LL) and 30–40% less during the dark period (DL). When photoperiod was inverted and leaves were measured for Kleaf at their subjective light or dark periods, Kleaf adjusted to the new conditions requiring 48 h for increasing from dark to light values and 4 d for the opposite transition. Plants put in continuous dark showed Kleaf oscillating from light to dark values in phase with their subjective photoperiod indicating that Kleaf changes were induced by the circadian clock. Several cuts through the minor veins reduced leaf hydraulic resistance (Rleaf) of both LL and DL to the same value (1.0 e + 3 MPa m2 s kg−1) that equalled the vascular resistance (Rv). The contribution of the non-vascular leaf resistance (Rnv) to Rleaf was of 71.9% in DL and of 58.4% in LL. The dominant Rnv was shown to be reversibly modulated by mercurials, suggesting that aquaporins play a role in diurnal changes of Kleaf.