The effect of the Na/Ca ratio in the growth medium on the response of the salt-sensitive crop species Oryza sativa L. to NaCl salinity was investigated. No effects on growth or salt concentration in the shoot were found over a range of Na/Ca ratios (5 to 25) considered likely to be commonly encountered in saline situations in the field, and even at extreme Na/Ca ratios (100 to 500) there were only marginal increases in NaCl entry to the shoot. Increasing the Ca concentration in the growth medium over a wide range did not, therefore, ameliorate the effects of salinity on rice seedlings. Polyethylene giycol at low, osmotically negligible, concentrations reduced the movement of CI to the shoot by 60 to 70% even in the presence of adequate (3.5 mol m−3) Ca. It is concluded that membranes of rice roots respond weakly to Ca and that they are capable of better permeability control than is evoked by Ca ions. The relevance of this to the salt sensitivity of rice is discussed.