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Three Rs

Replacement, Reduction, Refinement


Replacement refers to methods which avoid or replace the use of sentient animals in a study where they would otherwise have been used. These methods can be both absolute replacements (i.e. replacing animals with non-sentient systems, such as computer programs or cell cultures) and relative replacements (i.e. replacing sentient animals with animals that current scientific evidence indicates have a significantly lower potential for pain perception, such as some invertebrates). This may include using:

  • information already gained (e.g. systematic reviews and meta-analyses)
  • physical and chemical analysis techniques
  • mathematical and computer models
  • in vitro systems
  • human volunteers and human-oriented epidemiological approaches
  • invertebrates with less neuro-physiological development

One area where replacement techniques have proven particularly effective is toxicological regulatory testing as many traditional regulatory tests now have validated non-animal counterparts.


Reduction refers to any strategy that results in fewer animals being used to obtain sufficient data to answer a research question while maximizing the information obtained per animal. This potentially limits or avoids the subsequent use of additional animals although it is important that this be done without further compromising any individual animal’s lifetime welfare.

Global reduction in animal numbers can be achieved by reducing the number of animal experiments conducted, and by reducing the number of animals needed in each experiment. Careful experimental planning can help to achieve both aims. Additionally, reduction can be realized through sharing animals, tissue, or data, possibly across different studies, in such a way that it maximizes the information gained per animal.


Refinement refers to any modifications to husbandry or experimental procedures that minimize pain and distress for an animal. Because it is essential to consider the entire lifetime experience of the animal and not just its time spent during a procedure, refinement also refers to welfare-enhancing changes made to the animal’s living area. These changes are typically referred to as environmental enrichment, and scientists regularly work to implement effective enrichment strategies to realize the Three Rs.

Refinements that improve animal welfare can also improve the quality of research findings. For example, pain and distress are likely to result in physiological changes that may increase the variability of results and/or decrease their validity. By contrast, making the animal more comfortable does not increase variability in the data. This is therefore a case in which what is best for animals is also best for science.