I. Using a new method in which fishes were kept in their natural circumstances the metabolic rate of these was established by means of the diaferometer-technique. 2. The relation between the size of fish and the rate of gas-exchange per unit of body weight could be affirmed. The surface-area-law is discussed. 3. The influence of temperature is studied; the Q, 10 has probably a maximum value at optimum temperature. 4. The influence of CO2- and O2-pressure is discussed. Most fishes did not react at i oo % O2; at lower pressures a higher metabolic rate may be found, due to increased fidget of the animals. 5. Salt concentration probably is more important than pH of the water as to metabolic rate of fishes. 6. No results were obtained as to the influence of thyroxin and progesteron, added to the water, on the metabolism of Rhodeus amarus L.