1. The inhibition of oxygen consumption produced by a series of concentrations of ethyl carbamate has been measured in the protozoan Tetrahymena geleii.

2. The relation found between the narcotic concentration and its effect on respiration leads to the conclusion that urethane has two distinct modes of action in this cell. The respiratory data can be accurately predicted by assuming that the inhibitor acts on two independent parallel respiratory systems.

3. Complete suppression of cell division in this organism is brought about by approximately 0.1 M urethane.

4. Urethane concentrations up to 0.1 M affect primarily only one of the two postulated respiratory systems. The mechanism of the narcosis of cell division in this organism by urethane thus appears to be inhibition of this "activity" system.