The uptake of ammonium and nitrate by eelgrass (Zostera marina L.) was studied in two-compart-9 ment chambers. The plants were collected in 1992 from a population growing on a tidal flat in the S.W. Netherlands. They were incubated under conditions which reflected field conditions; this implied the use of natural seawater and sediment porewater as incubation media. In all six experiments, carried out over the course of a major part of the growing period (from July to the end of September), ammonium appeared to be much more important as a source of nitrogen than nitrate. The largest part was taken up by the leaves: uptake of ammonium by the leaves accounted for 68 to 92% of total plant nitrogen uptake. The uptake of nitrogen compounds by the rootrhizome system represented only 4 to 30% of total plant uptake. Thus, at least during flood fide, the leaves play the major role in nitrogen uptake in this intertidal population. During ebb tide, most of the plants are submerged in very shallow tidepools. It is suggested that during this phase of the tidal cycle, influx of porewater ammonium into the tide-pool water may enable the leaves to exploit local sediment resources.