1. The oxygen consumption in Paramecium caudatum and Paramecium aurelia is partially inhibited by potassium cyanide.2. The extent of inhibition by cyanide is dependent upon the food content of the organisms as well as upon the concentration of cyanide in the solution.3. In P. aurelia, starved specimens are insensitive to cyanide; old specimens are not as sensitive as young. In 10-4 M KCN respiration in the old organisms was inhibited by approximately 22 per cent while in the young organisms it was inhibited by approximately 40 per cent.4. In Paramecium caudatum, starved specimens were non-sensitive to KCN; old specimens exposed to 10-3, 10-4, and 10-5 M KCN show, respectively, a 42, 33, and 15 per cent inhibition in respiration. Young specimens, exposed to 10-3, 10-4, and 10-5 M KCN show, respectively, a 66, 42, and 30 per cent inhibition.5. The inhibition in the rate of respiration in P. caudatum was greater in buffer solution plus dextrose (0.01 M) than in the same solution without dextrose.6. The effec...