1. Osmotic pressure and chloride concentration have been determined for the coelomic fluid, blood and urine of earthworms kept in various saline media.
2. About 50% of the osmotic pressure of the coelomic fluid and of the blood can be accounted for as chloride. The blood is very slightly hypotonic to the coelomic fluid.
3. As the concentration of the medium is increased the osmotic pressure of the body fluids also increases and is always greater than that of the medium; the chloride increases proportionately, but is less than that of the medium when the latter exceeds 0.35% NaCl.
4. The urine is always strongly hypotonic to the body fluids except possibly in the most concentrated media (over 1.0% NaCl).
5. The osmotic relations of the earthworm are such as are characteristic of fresh-water animals generally.