For any fellow C. S. Lewis readers out there, perhaps you'd be as surprised as I was a few years ago to learn that he authored an essay against animal experimentation. "Vivisection," which you can read online and also find in his collection God in the Dock: Essays on Theology and Ethics (ed. Walter Hooper, Eerdmans '70), was originally published as a pamphlet by the New England Anti-Vivisection Society in 1947, whose president had requested Lewis write the tract after reading Lewis' thoughts on animal pain in The Problem of Pain.
In "Vivisection," Lewis challenges the critique of sentimentality which is levied against those who oppose animal experimentation, saying that the more imporant question is whether the practice is morally justifiable in the first place. Those who defend it assert that experimentation is necessary to prevent and relieve human suffering, but the pain inflicted (which is real) on animals remains in need of justification. Even if we suggest, as many Christians do, that it doesn't matter because animals do not have souls, and thus no conscience, this would make the pain even more unwarranted, because they could have done nothing to deserve it, and won't even have Heaven waiting for them in return. (By the way, see Dean Ohlman's excellent discussion of the question whether animals have souls in a previous post.)
Lewis suggests that our only possible defense of vivisection as Christians is that were made higher beings than them by our Creator, and therefore entitled to sacrifice them on behalf of our own good. After all, Matthew 10:31 indicates: "you are worth more than many sparrows" (NIV). But whereas Lewis doesn't challenge this "hierarchical order created by God" (which is in fact hard to challenge biblically, taking into account the fact that only humans are said to be made in God's image), he does astutely ask whether this means we can use them however we wish:
We may find it difficult to formulate a human right of tormenting beasts in terms which would not equally imply an angelic right of tormenting men. And we may feel that though objective superiority is rightly claimed for man, yet that very superiority ought partly to consist in not behaving like a vivisector: that we ought to prove ourselves better than the beasts precisely by the fact of acknowledging duties to them which they do not acknowledge to us.
Perfectly put. We don't answer to a "survival of the fittest" mentality, and we don't kill other species simply because we can. Our humanness includes a higher calling of stewardship, which at a minimum implies a higher bar set for determining the necessity of experimenting on animals, and a much higher standard of animal welfare when we reluctantly meet that bar.
Lewis doesn't deny that it might be still be possible for a Christian to in good conscience perform animal experimentation with a high sense of accountability and great care at every turn to prevent "the least dram or scruple of unnecessary pain," though Lewis himself doesn't seem to completely sign off on even this prospect. He is especially concerned in the remainder of the essay with the implications vivisection might have for society's openness to engaging in experimentation on members of our own species: "Once the old Christian idea of a total difference in kind between man and beast has been abandoned, then no argument for experiments on animals can be found which is not also an argument for experiments on inferior men."
On the whole it's a wonderful essay, from a Christian voice for animals long before his time (if that time has even arrived today). While Lewis raises an important and longstanding concern about how our poor treatment of animals inevitably affects how we treat each other as human beings, I appreciated most of all his brief attention to making sure we relate to animals in a manner which reflects our being made in God's image. As Andrew Linzey has pointed out: "Our metaphyisical privilege consists in being set in such a relationship to the earth and its creatures that we are required (as part of our very humanity) to exercise God-like care, even of a costly and sacrificial kind. There is no Christian privilege that doesn't involve service" (Why Animal Suffering Matters, pg. 33-34).
By the way, you might also be interested to learn that Linzey wrote an article on "C. S. Lewis's Theology of Animals," which I reviewed for one of my last seminary classes. If you're interested in a thorough overview of what Lewis wrote on the subject, more than I expected for sure, I heartily recommend Linzey's article. Apparently, Lewis also touches on the theme of animal experimentation some in his Space Trilogy novels.