The objective of this study was to compare the effectiveness and response of weaned piglets to 100% CO2 gas relative to a 50:50 CO2:Argon gas mixture as an effective tool for euthanasia. A total of 180 piglets, BW 4.6 ± 0.7 kg, were utilized. Piglets were 16 to 24 days of age. Two gas mixtures (100% CO2 and 50:50 CO2:Argon) and 4 flow rates (slow, medium, fast, and prefill; 20%, 35%, 50%, and prefill with 20%, chamber volume per minute,respectively) were examined. Two piglets were placed in a modified Smartbox™ (Euthanex Corp, Palmer, PA)chamber, in which the lid and one side are composed of clear plastic to facilitate behavior observations. Piglets were scored using direct observation for latency to perform three behaviors associated with insensibility: loss of posture, last movement and gasping. Open mouth breathing occurred prior to insensibility and was used as an indicator of distress. The CO2:Argon gas mixture and slow flow rates prolonged the duration of insensibility, as measured by last movement and did not confer advantages for measures of distress.