In concentrations of 1:104, 1:105, 1:106, in alkaline solution with a hydrogen ion concentration approximating that characteristic of normal human blood, thyroxin acts like total thyroid in accelerating certain metabolic processes in Paramecium, notably those connected with the phenomena of excretion. Unlike total thyroid, thyroxin depresses the division rate of Paramecium, the degree of depression varying directly with its concentration. This depression of the division rate is due to the action of the thyroxin directly on Paramecium, not to a modification of the food supply.