Even though the 3Rs are widely accepted as ethical standards when evaluating research projects using animals as experimental subjects, the ethical status of the 3Rs still remains to be clarified. The 3Rs were not derived from any ethical theory, but they represented an attempt to increase humanity to animal experimentation and at the same time to improve validity of scientific data . The aim of the present article was to provide an ethical analysis of the 3Rs through Engelhardt's bioethics theory (...) . The analysis revealed the 3Rs fitted to some extent Engelhardt ethical values. But the analysis also revealed some internal contradictions between the 3Rs and Engelhardt's ethical vaules. Although the 3Rs are still valuable tools in animal ethical evaluation, the present analysis suggests that some ethical issues still remain to be clarified. (shrink)