Breathing 79% nitrous oxide (N2O) in oxygen increased the rate of accumulation of bowel gas during intraluminal bowel segment infusions of hydrogen, methane (CH4), air, or carbon dioxide (CO2) in four pentobarbital-anesthetized dogs more than did breathing 100% oxygen. A N2O-associated increase in the volume of naturally produced intestinal gas in five halothane-anesthetized ponies corroborated the findings in the dog studies. In a second group of four dogs a bolus of CH4 or CO2 was injected into the bowel lumen. When the dogs breathed O2 the bowel gas volume decreased. Gas was virtually absent in the CO2-containing segment within 20 minutes. Breathing N2O increased the volume of the segments containing CH4 while the CO2 segments decreased less rapidly than during O2 breathing. Breathing O2 after 30 minutes of N2O breathing caused'little change in the rate of decrease in CO2 segment volumes. However, the CH4 segment volume ceased to increase and eventually-returned toward control volumes.