Measurements of colloid osmotic pressure and refractive index of five species of teleost fishes are recorded. A number of observations were made on eels which had been starved for two months. With the exception of the eel, the colloid osmotic pressures were less than half the characteristic level for mammals, the average being about 100 mm. H2O. Normal eels showed serum colloid osmotic pressures within the mammalian range, the starved eels showed values less than half as great but showed no signs of oedema.

A comparative table of colloid osmotic pressures for the vertebrates studied so far is presented.

The relation between refractive index and the colloid osmotic pressure is discussed briefly.

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