Animal research in the United Kingdom is regulated by the Animals (Scientific Procedures) Act 1986, which requires a government minister to weigh the expected suffering of animals against the expected benefits of a proposed animal research project—the “cost-benefit assessment”—before licensing the project. Research into the implementation of this legislation has been severely constrained by statutory confidentiality. This paper overcomes this hindrance by describing a critical case study based on unprecedented primary data: pig-to-primate organ transplantation conducted between 1995 and 2000. It reveals that researchers and regulators significantly underestimated the adverse effects suffered by the animals involved, while overestimating the scientific and medical benefits likely to accrue. Applying dynamic policy network analysis to this case in the context of the evolution of animal research policy indicates that an elitist, policy community type network has persisted since shortly after the network’s formation in 1876. Animal research interests have repeatedly withstood pressure for change from animal protection groups because of their greater resources, structural advantages, and a culture of secrecy that facilitates an implementation gap in animal research regulation.