Estuarine and montane plants of Armeria maritima (Mill.) Willd., differ slightly in anatomy. In their natural environment estuarine plants are frequently subjected to substantial variation in the total salinity of the root medium, while montane plants are not. Salt regulation in the two races was studied comparatively, using an electrochemical approach. The plants were grown in the laboratory in dilute culture, and subsequently exposed to increasing salinity up to the concentration of artificial sea water, the relative cationic composition of which was varied. The electrical potentials between the leaf water free space (W.F.S.) and the bathing media were measured, as were concentrations of the major ions in the W.F.S. and the bathing media, and the data applied to the Nernst Ussing-Tcorell equations. Comparison of predicted with actual fluxes obtained using radio-tracers showed active expert of Na+ and uptake of Cl- . K+ appeared to be actively imported at low external concentrations and passively distributed, with the possibility of active export at high and very high ambient concentrations. On the basis of purely electrical studies, a hypothesis in proposed for the role of the root in controlling the internal ionic environment of these plants with changes in the concentration of the bathing medium. The mode of action and efficiency as de-salinators of the salt glands present in the leaf are considered.