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While it is natural to be concerned about reducing negative experience (such as pain) for animals, providing positive experiences is also important. In some cases positive experiences can actually reduce the impact of stressful events. For example Dr. Sylvie Cloutier found that rats that were tickled first actually exhibited less stress when they received an injection.
Transportation of Research Animals for the Purpose of Biomedical Research, Testing, and Education
USDA/APHIS Animal Welfare Program