1. It appears that exposures of Melanoplus differentialis and Melanoplus femur rubrum to a stream of pure carbon dioxide for various lengths of time up to about 30 to 40 minutes produce reversible effects. Further, the time to the first movement and to recovery after short exposure seems to be independent of the length of exposure. Longer exposures to the gas require longer periods for recovery. It follows that the rate of the first movement and of recovery per minute exposure to the gas, up to 30 to 40 minute exposures, decreases rapidly. Above this limit the decrease in rates is imperceptibly gradual or remains constant until the exposure is sufficiently long to kill. 2. At 5 to 24 hours exposure to pure carbon dioxide, the animals are irreversibly affected and finally die. They may or may not exhibit reflexes upon removal from the gas. 3. Characteristic behavior, due to the influence of carbon dioxide, is exhibited. 4. There are three interdependent factors influencing the exchange of gases between the animal and the air in an enclosed space, namely, the original per cent. of carbon dioxide; the original per cent. of oxygen; the original ratio of carbon dioxide to oxygen.