1. 1. The changes of water content, volume of inulin space and Na, K and Mg content in the foot muscle and hepatopancreas of gastropod molluscs Littorina littorea, L. saxatilis and L. obtusata acclimated to lowered salinity (14-16‰) were investigated. 2. 2. The molluscs have an inability to completely regulate cell volume, the hydration level of which increases sharply at the first period of hypotony and partly re-establishes itself during prolonged acclimation to dilute seawater. 3. 3. During adaptation of L. littorea to lowered salinity the role of ions (especially potassium) in the cell osmoregulation increases. There is an effective mechanism of retaining potassium in the cells of these molluscs, the ionic quantity not decreasing even in the first steps of hypotony, when there is a great loss of sodium and magnesium ions.