It is well known that growth of Suaeda salsa (L.) Pall. is stimulated by NaCl application. However, the physiological role of sodium in the growth of Suaeda salsa has not been elucidated. We thus examined the physiological role of sodium in the growth of Suaeda salsa grown in nutrient solutions containing 5 mmol L(-1) Cl(-) and graded concentrations of Na(2)SO(4). Sodium concentrations in plant tissues increased as the growth medium salinity increased. The osmolality gradient between plant tissues and growth medium was maintained at constant levels despite the increase of osmolality in the growth medium. As a result, the amount of water absorbed by the plant tissues was not significantly affected by the salinity of the medium. The dry weight of the S. salsa plants did not increase significantly as sodium concentrations in plant tissues rose. As salinity of the medium increased, the contribution of water-soluble sodium to the osmolality in the leaves, stems, and roots also rose. There was a significant positive correlation between sodium concentration and osmolality in plant tissues. These results suggest that sodium is the major cation contributing to the maintenance of high osmolality in plant tissues of S. salsa grown under the high-salinity conditions. Therefore, under high-salinity conditions, the physiological role of sodium may be to maintain water absorption by maintaining osmolality in the plant tissues, which would help explain the normal growth of S. salsa under high NaCl conditions.