1. Nereis virens increased in weight on transfer to sea water diluted with fresh water. This increase was greater the greater the dilution. 2. The minimum survival concentration was between 30 and 40 per cent sea water. In concentrations of greater salt content than this, N. virens decreased in weight after the initial increase. This decrease was rapid for about a day and then gradual. 3. A few scattered observations of body weight changes which seemed to be associated with temperature changes have been made. These are in agreement with the report of Adolph, E. F. and P. E. (1925). 4. On return to sea water from a dilution, N. virens lost weight rapidly. This loss was greater the lower the concentration in which the animal had been kept. After this initial loss a slight increase in weight occurred. 5. When specimens of N. virens were placed for a second time in their respective dilutions they increased in weight again. This time, however, the changes were more gradual and smaller than those after the first subjection to dilutions. 6. Although N. virens survived but a few hours after direct transfer to 1/8 sea water, it lived 14 days in that dilution when transferred slowly through intermediate dilutions. 7. Worms which had been several days in dilute sea water lived in fresh water longer than did worms which had been transferred directly from ordinary sea water. Also the more dilute the solution in which the animal had been kept the less the rise in weight on transfer to fresh water.