The effect of different concentrations of NaCl, mannitol and CaCl2 on the retention of potassium ion by preloaded roots of barley, Hordeum vulgare, and bean, Phaseolus vulgaris, indicated that 50 mM NaCl had no effect on potassium loss, but 50 mM NaCl or more increased the rate of efflux. Barley roots appeared 10 be more sensitive to salt stress than bean roots since 100 mM NaCl caused 78% potassium loss over 6.5 h by barley as compared with 40% loss by bean roots. Mannitol up to 200 mM had no effect on the retention of potassium by barley roots but slightly increased the potassium loss from bean roots (26%). The results suggested that salt stress is more effective than osmotic stress in disrupting the membrane permeability and calcium ions can prevent the increased permeability caused by osmotic or ionic stress.