Xylem pressure in young barley roots, measured in vivo with a xylem pressure probe, showed constant, irregular fluctuations both under altered or unchanged environmental conditions. When mild salt stress was applied or when the stress was eliminated, xylem pressure in barley roots exhibited intense self-regulation or relaxation, leading to a consequence that the difference of xylem pressure before and after the salt stress was greatly narrowed and the barley plants could maintain a relatively stable xylem pressure. The process of regulation or relaxation of xylem pressure in barley roots lasted about one hour or more before a relatively stable state was achieved. The self-regulation or relaxation of xylem pressure was not dependent on the initial status of the xylem pressure. Both the fluctuation and the self-regulation of xylem pressure in plant roots could exert substantial uncertainties on the conventional measurements of water potential and the radial reflection coefficient of barley roots, especially when the pressure bomb (Schollander bomb) or root pressure probe was used, because the pressure was in constant change and the measurements of some data points could not reflect the true situation within plants. The radial reflection coefficients of barley roots, when subjected to NaCl stress, were in a range from 0.56 to 0.97 if the maximum responses of the plants to stress were taken. However, if the pressure responses after the self-regulation were adopted, the coefficients fell in between 0.24 and 0.32 for the same plants. Such results could have two implications. On the one hand, care had to be taken when the radial reflection coefficients of roots were determined, because the points where the data were taken could greatly alter the final results. In this case, it could be necessary to specify the type of the results, i.e. the maximum response or the regulation-diverted response of plants to stress and hence the resultant radial reflection coefficients of roots. Also, care must be taken when the data were used in the interpretation of the physiological processes in terms of plant response to salt stress. On the other hand, the self-regulation or relaxation of xylem pressure in barley roots, which prevented great variations in xylem pressure in plants, could be beneficial to plants under salt stress and could as well be related to the mechanisms of salt resistance in plants. Large self-regulation or relaxation of xylem pressure under different environmental conditions was not an universal phenomenon in plants because it was not seen in wheat roots before and after the application of salt stress.