Animal welfare legislation is placing increasing emphasis on researchers to demonstrate competence in all procedures that their research may require, even if some are infrequently performed. Many field biologists rarely need to carry out euthanasia, but they may nevertheless require a legal, transportable and reliable method to be available for them so that they can obtain the necessary approvals to undertake their work. A small CO2 system, reproducing the recommended method used widely in laboratories, seemed likely to meet these requirements. Three sources of CO2 that could be taken into field situations were tested: soft drink carbonating machines; the chemical production of CO2 by mixing sodium bicarbonate and acetic acid; and, small disposable CO2 cartridges. Of these, a system designed to use CO2 cartridges was found to be the most reliable and practical. This new equipment was tested for efficiency against a conventional, standard laboratory CO2 euthanasia system using laboratory mice (8) and rats (6) that were due for euthanasia. No statistical difference was found between the two systems in the time taken for either species to succumb to CO2. This new equipment therefore meets welfare guidelines and its small size, compact nature and simplicity to operate make it highly suited to use in the field. Finally, because cartridges are available in a variety of sizes, it can be readily adapted and scaled to meet the needs of different taxa.