High concentrations of carbon dioxide (CO2), used for killing laboratory rodents, are known to be more strongly aversive to rats than sweet food items are attractive. This study investigated whether the maintenance of a high oxygen (O2) concentration, using a gas
mixture of 70% CO2 and 30% O2, would reduce aversion to CO2 during a gradual-fill procedure. Eight male Wistar rats, aged 10 months, were housed individually in an apparatus consisting of two cages, one higher than the other and joined by a tube. In a series
of trials, subjects entered the lower cage for a reward of 20 sweet food items. The gas was turned on at the moment the rat started eating the reward items and flowed into the lower cage at a fixed rate. There were four treatments: 1) 100% CO2 at 14.5% cage volume min–1;
2) gas mixture at 14.5% min–1; 3) gas mixture at 21.0% min–1, which delivered CO2 at approximately 14.5% min–1 and 4) air, with each subject tested with each treatment four times. Measures of willingness to stay and eat in the lower
cage (latency to stop eating, latency to leave and the number of reward items eaten) were much lower in all three gas treatments than in air, indicating that the CO2 and the CO2 + O2 mixture were both more strongly aversive than sweet food items were attractive.
Comparing the gas mixture with 100% CO2, the latency to leave and the number of reward items eaten were slightly higher in the CO2 + O2 mixture at 21% min–1 than in CO2 at 14.5% min–1, indicating that the addition
of O2 slightly reduced the aversiveness of CO2 in the gradual-fill procedure. This reduction is not enough to warrant recommending the use of CO2 + O2 mixtures for killing rats.