An enclosed gas-filled space in the body will expand if gas within it is less soluble than the gas respired. Blood arriving at such a space can discharge a greater quantity of the soluble gas into the space than that blood can take up, assuming the tension gradient of each gas is equal. This results from the greater capacity of blood for the more soluble agent. When air was placed in the intestinal lumens of 3 dogs and nitrous oxide respired, intestinal gas volume increased 75 to 100 per cent in two hours and 100–200 per cent in four hours. Similarly, 300 ml. of air placed in the pleural space doubled in volume in 10 minutes, tripled in 45 minutes, and in one dog quadrupled in two hours. Nitrous oxide concentrations rose concomitantly in both the intestinal and pleural spaces. With cither gas in the intestine or in the pleural space, no volume changes were seen when the animal respired oxygen and halothane alone. These results suggest that nitrous oxide is relatively contraindicated in cases of intestinal obstruction or pneumothorax.