AbstractUptake and transport of Na and K was studied using the radioactive tracers22Na and42K in intactAster tripoliumL. seedlings grown at two salinities CS 10 and CS 100, (containing 10mol m−1and 100 mol m−3Na, respectively, together with other major ions in the proportions found in sea water). At both salinities a much greater proportion of the Na than K taken up by the plant was subsequently transported to the shoot. Most42K fluxes were reduced by about 40% in CS 100 plants relative to CS 10 except root accumulation which increased. Experiments involving changing the salinity from CS 10 to CS 100 showed that42K fluxes remained constant for at least 40 h, indicating that competition with Na for uptake sites was not the cause of the reduced flux in CS 100 plants.22Na fluxes responded immediately to a change in salinity with all fluxes increasing six‐fold when the salinity was raised. When the salinity was lowered, however, root accumulation returned to the level in CS 10 control plants whereas transport to the shoot was inhibited by the previous high salinity treatment, being reduced to only 35% of the rate in CS 10 plants. The time courses of osmotic adjustment and Na accumulation following an increase in salinity were found to be very similar, with sufficient Na being accumulated to account for the observed increase in sap osmotic pre