Work into treating serious diseases is being threatened by animal rights campaigners, say scientists.
Around 15,000 animals - mostly mice - are brought in from abroad every year for testing new medicines.
But pressure from activists means ferries and airlines are refusing to carry animals heading for labs - meaning fewer animals for research.
Campaign group Peta says 90% of drugs that pass animal trials end up not working on humans.
They think researchers should work harder to find alternatives.
But supporters say testing on animals is necessary because it helps to save people's lives.
What do you think about animal testing? Is it important to help save human lives? Or do you believe it's cruel to use animals for experiments?
Here are some of the comments you sent in.
"I think it should be fair to test animals only if it is a cure for humans, because my aunty died last year from breast cancer and I would like a cure for that."
Benjamin, 9, Southport, England
"Testing on animals has its advantages and disadvantages. We could find a cure for bad diseases that people die from, but the animals haven't done anything wrong."
Paolo, 13, Hertfordshire, England
"Everyone says that they have as many rights as us but would you rather have some animals you don't know being tested or humans such as your parents?"
Josh, 13, Cornwall, England
"I think animal testing is WRONG! Why would anyone want all these new tests put into you? There should be a BIG FULL STOP to this horrible thing!"
Emily, 9, Bedfordshire, England
"I think it's totally right - yes I feel sorry for the animals but without animal testing they could be releasing dangerous drugs onto the market, both for animals and humans."
Joe, 12, Surrey, England
"Animal testing is wrong but it's helped us accomplish a lot..."
Emily, 12, Rotherham, England
"I think it's fine for us to test on animals because we will be able find cures for ill people just by testing on a few tiny creatures. That's not too bad, is it?"
Casey, 11, Sheffield, England
"I disagree extremely with anyone that says it's right to test on animals. Who is to say that all those animals' lives are worth more than a human's? Animals should have equal rights as humans because they are living breathing creatures as well."
Ben, 12, Craigavon, Northern Ireland
"I can't say that I disagree with animal testing because it's not only saved my life but other people in my family and my friends. It is inhumane but the amount of lives it saves is incredible."
Phoebe, 12, Warwickshire, England
"I REALLY hate people who test on animals! I have a lot of pets at home and the thought of anyone testing on them makes me sick!"
Emily, 12, Kent, England
"I think animal testing is cruel and unfair... how would you like it if someone used you to carry out medical investigations resulting you losing your life?"
Anna, 13, Lancashire, England
"I think animal testing is really wrong because who's to say that humans are better and we can kill innocent animals just because we are apparently a higher rank than them, which isn't true! Why don't the scientists test it on themselves instead of being mean and testing on innocent animals who can't speak for themselves!"
Georgina, 12, Yorkshire, England
"I think it's right to do because if we didn't test the cures we would never know if they work."
David, 8, Aberdeenshire, Scotland
"It's not fair to the poor animals but on the other hand, if we test them we will save people from bad things like cancer!"
Paige, 9, Surrey, England
"Although results in mice may not reflect the results in humans, I don't think animal testing is a bad thing. Having someone die from an untested medication is worse than having a mouse die from it. Although I do admit all life is precious."
Rob, 14, Burton, England
"I thinks it's right that we test on animals, a few mice for one million humans!"
India, 12, Belfast, Northern Ireland
"It's wrong to test animals if it's something that we already know how to make, like shampoo. But if it means it's going to cure cancer or something that will affect lives then it's right."
Maya, 11, Lancaster, England
"I think that it's fine because if we only lose a few or so mice, many lives could be saved. However, I do feel it's inhumane - but that's life!"
Katie, 10, Southampton, England
"I think it's wrong to test drugs on animals as they can be very harmful and it's against the animal's rights."
Grace, 11, Dundee, Scotland
"I think that animals should have the same rights as humans. If we treat our pets as our friends then we should treat other animals like that as well."
Oli, 11, Hampshire, England
"I think that animals have not done anything to us so why should we do stuff to them?"
Sean, 10, South Yorkshire, England
"I don't see the point in testing on animals because we have different bodies."
Jasmine, 14, Greater London, England
"I think they need to use alternative ways to test medicines as testing them on animals is cruel."
Terri-Jane, 12, Devon, England
"I think animal testing is bad and I would do anything to see it end. Imagine if it was your dog or cat or any other pet, would you risk its life to save yourself just in case you might get an illness in the future?"
Phoebe, 12, Northamptonshire, England
"I think it's wrong because animals should have as many rights as we do. They feel things too. They die for no reason. Not all the medicines work on humans in the same way!"
Megan, 12, England
"I believe animal testing is right, we could could find a cure for cancer and save millions at the expense of a few mice!"
James, 13, Monmouthshire, Wales
"I feel sorry for the poor animals, but if we don't test on animals we wont find new cures."
Tomas, 14, Spain
"It's inhumane to do something so wrong and cruel to animals! They are living creatures just like us!"
Joanna, 11, London, England
"I think researchers should find alternatives for research because not every product that passes in animal testing works on humans."
Claudia, 14, Jakarta, Indonesia
"I think it is wrong to test on animals as they have rights and feelings too. I understand why people test on animals… but we should do it on animals that aren't dying out and be careful how much testing we do. Also instead of testing on animals, scientists could use clones of human skin/cells."
Kay, 15, Lancashire, England
"It's wrong to test on animals, there's always a better option. It may take time to find it, but there's always one."
Charlotte, 14, Wrexham, Wales
"I think it is wrong to use animals for testing for new drugs and medicines. There are other ways to test, just don't use animals."
Asim, 14, London, England