Prefill 90% nitrous oxide (N₂O) was not aversive and piglets succumbed to the gas.

Prefill 60% N₂O with 30% O₂ did not cause aversiveness or unresponsiveness.

In contrast, 90% CO₂ was highly aversive, causing flailing behavior.

Both 90% N₂O and 90% CO₂ induced isoelectric EEG in 71 and 59 s respectively.

N₂O is much less aversive than CO₂, and 90% N₂O can euthanize piglets.