Changes in the root diameter were measured in wheat, Triticum aestivum L., using a sensor of small displacements in "osmotic experiments," and the hydraulic conductivity (L_p) of the root was calculated by the method of initial flows. The L_p decreased by two to three times during 30-mm exposure m 0.1-9 mM H₂O₂. In 150 mM NaCl, changes in L_p exhibited two-phase kinetics: an initial increase in L_p by two and a half to three times, followed by a decrease to 1.6-50% of the control level within the subsequent 24 h. After one-day-long exposure of the roots in a solution containing 150 mM NaCl and 100 mg/l catalase, L_p did not differ from the control level. Root pretreatment with salicylate, which promotes the formation of H ₂O₂, prevented the initial increase in L_p in 150 mM NaCl, probably, due to a drastic L_p decrease. The activity of guaiacol peroxidase in the roots increased by three times within the 20-min exposure of the roots to the salt solution. The data suggest that hydrogen peroxide can mediate the L_p decrease during the first day of salinity stress.