Abstract A method is described for perfusing xylem vessels in tap root segments of the halophyte P. maritima. Use of excised segments allowed recording of the trans-root potential (TRP) at both ends of a segment. It was shown that there can be a spatial variation of electrogenic ion pump activity along the xylem in one root segment. The pH of perfusion solutions, differing in buffering capacity, was adjusted by the root segment to pH 5.1–5.6 during How through the xylem. This pH range was similar to that of sap produced by root pressure. The K+ activity in the outflow solution (K+out) was rather constant at 12–13 mol m−l3 despite input K+ activities ranging from 8 to 20 mol m−l3. Addition of fusicoccin (10−l2 mol m−l3) to the perfusion solution induced a strong acidification of the xylem sap, a decrease in K+out and an increase in Na+out. Inhibition of aerobic respiration through anoxia inhibited electrogenic proton pumping into the xylem and led to an increase in K+out and a decrease in Na+out. It is suggested that transport of K+ and Na+ to the shoot of the halophyte P. maritima is regulated in the tap root by means of ion exchange between xylem vessels and xylem parenchyma and that this exchange is energized by proton translocating ATPases.