Modifications of vessel size and of shoot hydraulic conductivity induced by different water availability levels (?leaf -0.35 MPa -0.6 MPa and -0.8 MPa respectively) were investigated in container-grown grapevine plants. Plant water loss measured as xylem sap flow and as leaf transpiration was lower in waterstressed plants. Morphometric measurements on xylem showed that vessels of water-stressed plants had lower transectional areas. Shoot hydraulic conductivity (kh) shoot specific conductivity (ks) and leaf specific conductivity (kl) were lower in water-stressed plants. When conductivities were measured on shoot portions differences between treatments were particularly high at the basal internodes. At the lower stress level no embolism was detected and reduced conductivity could be explained by the reduction of vessel diameter according to the Poiseuille equation. At the higher stress level kh was further reduced by formation of vessel embolisms. The tension gradient along the shoot increased only at the higher stress level. Stomatal conductance was linearly correlated with kl at low stress levels suggesting a concerted regulation of water flow while at higher stress levels stomatal conductance decreased with no changes of kl. Reduced development of xylem vessels in grapevines subjected to moderate water stress may contribute to the control of water flow and to a reduction in vulnerability to xylem embolism.