| Summary: | The swine industry is often forced to euthanize pigs in the first few weeks of life due to injuries, hernias, or unthriftiness. The majority of pigs are euthanized using carbon dioxide (CO<sub>2</sub>) gas asphyxiation but concerns as to the humaneness of CO<sub>2</sub> are increasing. This study compared the euthanasia of weaned pigs using N<sub>2</sub>O (N<sub>2</sub>O; <i>n</i> = 9) or CO<sub>2</sub> (<i>n</i> = 9), at 50% and 25% min<sup>−1</sup> exchange rate, respectively. In addition, we administered an analgesic prior to euthanasia with CO<sub>2</sub> (CO<sub>2</sub>B) exposure as a third treatment (<i>n</i> = 9) to elucidate behaviors indicative of pain. Pigs in the CO<sub>2</sub> and N<sub>2</sub>O treatments lost posture at similar times (latency of 145.0 ± 17.3 and 162.6 ± 7.0 s respectively, <i>p</i> > 0.10), while the CO<sub>2</sub>B treatment pigs lost posture the soonest (101.2 ± 4.7 s, <i>p</i> < 0.01). The pigs in the CO<sub>2</sub>B treatment made more escape attempts than the CO<sub>2</sub> or N<sub>2</sub>O pigs (16.4 ± 4.2, 4.7 ± 1.6, 0.3 ± 0.2, respectively; <i>p</i> < 0.0004). However, pigs in N<sub>2</sub>O squealed more often than either the CO<sub>2</sub> or CO<sub>2</sub>B pigs (9.0 ± 1.6, 2.8 ± 1.2, 1.3 ± 0.6, respectively, <i>p</i> < 0.001). Given the similar time to loss of posture and shorter time displaying open mouth breathing, N<sub>2</sub>O may cause less stress to pigs; however, the greater number of squeals performed by these pigs suggests the opposite. It was not apparent that any behavior measured was indicative of pain. In conclusion, N<sub>2</sub>O applied at a 50% min<sup>−1</sup> flow rate can be an alternative to CO<sub>2</sub> for pig euthanasia.
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