Which option best defines Replacement in the 3Rs?

• A. Substituting non-animal methods for animals when feasible.
• B. Reducing the number of animals used.
• C. Replacing animals with fewer sentient species only.
• D. Substituting non-animal methods or less sentient organisms for animals when feasible.

Answer: (D)

Replacement means substituting methods that do not involve animals, or using organisms with lower sentience, for animal use whenever that substitution can achieve the same scientific aims without compromising validity. This includes non-animal approaches like advanced in vitro systems, computer models, and human-based studies, as well as using less sentient organisms such as certain invertebrates when appropriate and feasible. The emphasis is on moving away from animal use when possible, guided by scientific and regulatory feasibility. The other ideas are narrower: reducing the number of animals is about Reduction, not Replacement, and limiting substitution to fewer sentient species only misses the broader scope of Replacement, which also accounts for non-animal methods.