Exposure to CO(2) is a common method used to euthanize rodents in biomedical research and rodent production. The purpose of this study was to determine the length of CO(2) exposure required to euthanize neonatal rats (0 to 10 d old). Multiple groups of rats were exposed to 100% CO(2) for 5 to 60 min. After CO(2) exposure, rats were placed in room air for 20 min to allow for possible recovery. No difference was found in comparing 1 inbred strain and 1 outbred stock of rats. Time to death varied inversely with the age of the animals, requiring as long as 35 min on the day of birth. The time to death decreased steadily with increasing age, with 100% of the rats euthanized after 5 min of CO(2) exposure at 10 d of age. The time required for 100% mortality decreased by 3 min for every 1 d increase in age between days 0 and 10.