The effects of root infection by Phytophthora cinnamomi on stomatal conductance in Castanea sativa L. saplings were investigated to determine the potential role of root-derived chemical signals. A split-root experiment was carried out, in which inoculation of the pathogen or drought was applied to the root systems in either one or both compartments. At the end of the experiment plant sap extracts were collected and their effects on stomatal conductance were determined by leaf bioassay. Inoculation or drought imposed in both compartments resulted in decreases in stomatal conductance (g(s)), transpiration rate, soil-to-leaf specific hydraulic conductance, leaf water potential, xylem [ABA] and root biomass, but not in the ratio of root-to-leaf mass in inoculated plants. Conversely, only gs and xylem [ABA] were affected in plants inoculated or droughted in one compartment, and no changes were detectable in leaf water potential and soil-to-leaf specific hydraulic conductance. The leaf bioassay showed that gs in chestnut was sensitive to ABA but not to Phytophthora elicitins. Stomatal conductance was reduced by some sap extracts, both from control and inoculated plants. Our results suggest the involvement of different signals, chemical and hydraulic, in regulating stomatal conductance of chestnut at different stages of stress