Diurnal variation in petiole specific hydraulic conductivity and simultaneous measurements of leaf water potential were recorded in red maple, tulip tree and fox grape. Petiole specific conductivity was determined from in situ measurements of water flow into the distal (leaf-bearing) end of an attached petiole as a function of applied hydrostatic pressure and petiole dimensions. The hydraulic properties of the petiole dominated the measurements, indicating that this technique can be used for rapid estimates of petiole hydraulic conductivity. There was a significant decrease in petiole specific conductivity associated with increasingly more negative leaf water potentials in maple and tulip tree, but not in grape. Petiole specific conductivity increased during the afternoon while the plant was actively transpiring and the xylem sap was under tension. The recovery of petiole conductivity during the afternoon suggests that hydraulic conductivity reflects a dynamic balance between a loss of hydraulic conductivity with increasing water stress, and its restoration as tension within the xylem decreases. Three experimental manipulations were applied to red maple and tulip tree to examine the sensitivity of diurnal changes in petiole conductivity to various physiological perturbations. Both phloem girdling and application of HgCl2 to the transpiration stream resulted in a marked decrease in the degree to which petiole specific conductivity recovered as xylem tension relaxed during the afternoon. Delivery of a surfactant to the xylem, however, did not significantly alter the relation between leaf water potential and petiole hydraulic conductivity.