All anaesthetics produce a reversible depression of central nervous system activity, and on occasion, the degree of depression may become excessive and the animal dies. This should be a very rare event when anaesthetising healthy laboratory animals. The death of an animal during anaesthesia should stimulate a review of the entire process of animal selection, choice of anaesthetic and pre- and intra-operative care. In people, anaesthetic mortality rates are approximately 34 per million. Mortality rates in veterinary clinical practice have been reported as 1:400 (in cats) and 1:600 (in dogs). When anaesthetising healthy, young adult laboratory animals, it does not seem unreasonable to expect a mortality rate of <1:1000. Good pre-anaesthetic preparations, as described above careful assessment of the physiological state of the animal during the anaesthesia can result in a dramatic improvement in recovery rates.