Determining ethical standards for laboratory animal euthanasia requires an assessment of the relative amounts of pain and distress caused by different methods. Animal behaviour data are an important indicator of pain and distress, but their interpretation can be controversial; moreover, behaviour is more easily assessed with some euthanasia methods than with others. While every euthanasia method requires careful study, CO2 inhalation has come under close scrutiny both because it is so widely used for rodent euthanasia, and because it has, until recently, long been considered relatively non-aversive.