Animal models are often used to test the effectiveness of a drug or procedure before proceeding to clinical trials. One reason for use of animal models is that they allow researchers to focus on particular pathological processes without the confounding effects of other injuries and treatments. However, it is essential that their results are valid and precise. Biased or imprecise results from animal experiments may result in clinical trials of biologically inert or even harmful substances, thus exposing patients to unnecessary risk and wasting scarce research resources. Moreover, if animal experiments fail to inform medical research then the animals suffer unnecessarily.

The Italian pathologist Pietro Croce criticised vivisection on scientific grounds. He argued that results from animal experiments cannot be applied to humans because of the biological differences between animals and humans and because the results of animal experiments are too dependent on the type of animal model used.1 Croce's arguments were based on insights into zoology and pathophysiology. In this paper, we make some methodological observations on animal experiments. Our observations were made in the context of a systematic review of all available randomised controlled trials of fluid resuscitation in animal models of uncontrolled bleeding. We conducted this review because we wanted to assess the scientific basis for fluid resuscitation. A previous systematic review of randomised trials of fluid resuscitation in bleeding trauma patients had provided no evidence that fluid resuscitation improved outcome.2