The Institutional Animal Care and Use Committee is entrusted with assessing the ethics of proposed projects prior to approval of animal research. The role of the IACUC is detailed in legislation and binding rules, which are in turn inspired by the Three Rs: the principles of Replacement, Reduction, and Refinement. However, these principles are poorly defined. Although this provides the IACUC leeway in assessing a proposed project, it also affords little guidance. Our goal is to provide procedural and philosophical clarity (...) to the IACUC without mandating a particular outcome. To do this, we analyze the underlying logic of the Three Rs and conclude that the Three Rs accord animals moral standing, though not necessarily “rights” in the philosophical sense. We suggest that the Rs are hierarchical, such that Replacement, which can totally eliminate harm, should be considered prior to Reduction, which decreases the number of animals harmed, with Refinement being considered last. We also identify the need for a hitherto implicit fourth R: Reject, which allows the IACUC to refuse permission for a project which does not promise sufficient benefit to offset the pain and distress likely to be caused by the proposed research. (shrink)