Measurements were made of net H(+) loss from low-salt barley roots accumulating salt. Comparison of rates of loss from roots in different concentrations of KCl showed that H(+) loss increased in the same way as the Mechanism II component of salt uptake. This H(+) loss appeared to be coupled to salt uptake and was not due to increased respiration or metabolic breakdown of sugars. In view of the large negative potential of the cells (-60 millivolts), it is suggested that the H(+) loss is due to an outward proton transport process.