1. A method for measuring the O2 consumption of a single cell is described. The cell is placed in a capillary tube adjacent to a bubble of air. KOH (5 per cent) is drawn in on the opposite side of the air and both ends of the tube are sealed with mineral oil. The decrease in the volume of the gas, representing the O2 consumed, is followed.
2. The possible errors of the technique are appraised.
3. A single Actinosphaerium eichhornii consumes 0.00113 mm.3 of O2 per hour. A single Paramecium caudatum consumes 0.00049 mm.3 of O2 per hour.
4. The significance of the results and the limitations of the method are discussed.