Russell and Burch’s 1959 original definitions of the 3Rs (replacement, reduction and refinement) are widely used today as standards for the ethical use of non-human animals in research, although they have a number of limitations. Authors and institutions around the world have addressed some of these, coming up in certain cases with more accurate, functional, and up-to-date definitions. However, not only do there still remain limitations needing to be addressed, but some that have been addressed resulted in discrepancies, contradictions, and general confusion as to how best apply the 3Rs in practice. In order to clarify the meaning of the 3Rs and enable more optimal implementation of these principles in animal experimentation, this article provides a theoretical discussion for revised definitions of the original 3Rs via examination of some of their main limitations and inconsistencies. First, we offer up the original definitions as presented in the context of Russell and Burch’s book The Principles of Humane Experimental Technique. Then, we examine the main limitations and present clear specifications and requirements for such revised definitions. After presenting our revised definitions, we conclude with various implications for animal welfare within the context of experimentation.

William Russell and Rex Burch's 3Rs principles were developed 1959 before animal ethics emerged as a scientific discipline in the 1970s and before many ground-breaking developments in modern biotechnology, such as genetic engineering. From this starting point we sought to analyse the normative foundations of the 3Rs principles in contemporary terms and concepts of animal ethics. After establishing the normative groundwork of the 3Rs, we will look at their practical implications within the context of present-day biotechnology. To this end, we shall investigate whether the genetic disenhancement of research animals to limit their ability to feel pain (GPD) complies with the original 3Rs principles. We use GPD as a practical example, since it is being discussed today as a promising way of solving one of the key moral issues raised by animal research, notably animal pain and suffering. By discussing GPD in the context of the 3Rs we aim also not only to gain insights into whether GPD is compatible with one or more of the 3Rs, but also to develop a better understanding of the specific normative foundation of the 3Rs principles and the conceptual limitations and practical implications of that foundation. We argue that reducing moral concerns about animal research to those that are intelligible within a sentientist framework (eg harm and suffering), as the 3Rs do, represents an oversimplification of the moral issues involved. We suggest that interference with abilities, instrumentalisation, flourishing, and death are all important aspects of animal ethics requiring consideration.