The pressure potentials of xylem vessels of the root system and the Na+ content of the xylem sap in soybean seedlings under salt stresses were measured with a xylem pressure probe and an atomic absorption spectrometer, respectively. The results showed that the negative pressures in xylem vessels increased, but the relative pressures and the radial refliection coefficient of root decreased with the elevation of salinity in the root environment. Although the Na+ content in the xylem sap increased with the elevated salinity levels in the root bathing solution, the relative amount of Na+ declined. Meanwhile, both the pressure and osmotic potentials in the root xylem declined with the increase in salinity, but the changes in the total water potential of the root xylem solution (osmotic potential plus pressure potential) resulted from salt stress were always smaller than the changes in the water (osmotic) potentials of the solutions bathing the root. In other words, the total water potential of the xylem solution was increasing steadily over the water potential of the root bathing solution with the increase in the intensity of salt stress. These results indicated that the soybean plants could use small negative xylem pressures to absorb water from salinized solution, and avoided the uptake of excessive amount of Na+ to combat saline stresses.