Yes, the vast majority of animals used in research and testing are killed when a study ends. They are euthanized either because their tissues and organs need to be examined, because they have reached a point where continued living would mean suffering, or because they are simply no longer needed. A smaller number of animals, particularly dogs, cats, and chimpanzees, may be retired or rehomed depending on the type of study and the laws in the state or country where the research takes place.
Why Most Animals Are Killed After Studies
The most common reason animals are killed after testing is that researchers need to look inside their bodies. In toxicology studies, for example, the FDA requires that all animals in a study undergo a complete necropsy, which is a detailed post-mortem examination. Every organ and tissue is inspected with the naked eye and then under a microscope. Researchers compare what they see in treated animals to what they see in untreated control animals, looking for signs of damage, tumor growth, or other changes caused by the substance being tested. This microscopic examination of tissues is the core safety data that regulators use to decide whether a food ingredient, drug, or chemical is safe for humans.
There is no way to do this while the animal is still alive. You cannot remove a liver, slice it into thin sections, stain it, and examine it under a microscope without killing the animal first. For brain studies specifically, certain methods of euthanasia are chosen because they preserve brain chemistry in its exact state at the moment of death, which allows researchers to measure what was happening in the brain during the experiment.
Other animals are killed because continuing to keep them alive would cause unnecessary suffering. Research protocols are required to define “humane endpoints,” meaning predetermined criteria for when an animal should be euthanized rather than allowed to deteriorate further. Animals with solid tumors, for instance, are killed when the tumor interferes with eating, drinking, or movement, or when they show signs like significant weight loss or lethargy. The decision to euthanize is supposed to be built into the study design before the experiment even begins.
A third group of animals is killed simply because they are surplus. Breeding colonies produce more animals than researchers need, or animals are born without the specific genetic traits a study requires. These animals are euthanized because there is no protocol for them and, in most cases, no system in place to rehome them.
Types of Studies Where Animals Survive
Not all research requires killing the animal at the end. Behavioral studies, vaccine efficacy trials, and some long-term nutritional studies can use non-terminal methods like small blood draws to track health markers over time. Techniques exist for collecting tiny blood samples (as little as 50 to 100 microliters from an ear vein) from conscious animals without harming them, allowing researchers to monitor the same individual animal across weeks or months. This approach actually produces better data in some cases because tracking one animal over time reduces the variability that comes from comparing different animals, and it means fewer animals are needed overall.
That said, even in non-terminal studies, animals are often euthanized eventually. A behavioral study might keep animals alive for the duration of the experiment but kill them afterward for tissue analysis that supplements the behavioral data. True survival to natural death is uncommon in laboratory settings.
Limits on Reusing Animals
One question people reasonably ask is: why not reuse the same animal for multiple studies instead of killing it? U.S. federal regulations under the Animal Welfare Act do place limits on this. No animal can be used in more than one major surgical procedure from which it recovers, unless the principal investigator provides written scientific justification, the procedure is a routine veterinary treatment, or the USDA grants special permission on a case-by-case basis. The reasoning cuts both ways. Reusing animals reduces the total number killed, but subjecting one animal to repeated invasive procedures raises its own welfare concerns.
Rehoming Laws for Dogs and Cats
A growing number of U.S. states have passed laws commonly called “Beagle Bills” that require research facilities to offer dogs and cats for adoption rather than euthanize them when studies are complete. These laws apply when the animal is healthy and poses no risk to public safety. California, Connecticut, Illinois, Maryland, Massachusetts, Minnesota, Nevada, and New York all have versions of this legislation.
Massachusetts passed its version in 2022, on the heels of a high-profile rescue of thousands of beagles from a breeding facility called Envigo. Under these laws, once a lab determines that a dog or cat is no longer needed for research, it must contact an animal shelter or rescue organization to arrange placement in an adoptive home, or pursue private placement directly. The laws specifically replace the default practice of euthanizing these animals when they “retire” from research.
Beagle Bills only cover dogs and cats. They do not apply to mice, rats, rabbits, guinea pigs, or other species, which together make up the overwhelming majority of animals used in research. Most rodents used in labs are not even counted under the federal Animal Welfare Act’s reporting requirements.
Chimpanzee Retirement Protections
Chimpanzees have the strongest legal protections of any research animal in the United States. The CHIMP Act, passed by Congress in 2000, established a federally funded sanctuary system to provide lifetime care for chimpanzees that were used in, or bred for, government-supported research. A 2007 amendment called the “Chimp Haven is Home Act” went further, permanently banning the removal of chimpanzees from the sanctuary system for any future research. Once a chimpanzee enters the sanctuary, it stays for life and cannot be discharged.
The federal government retains ownership of these chimpanzees, and the sanctuary is operated by a contracted nonprofit organization. These protections reflect both the cognitive complexity of chimpanzees and shifting public attitudes about using great apes in research. The NIH ended all biomedical research on chimpanzees in 2015.
The 3Rs Framework
The scientific community’s main ethical framework for animal research is called the 3Rs: Replacement, Reduction, and Refinement. Replacement means using non-animal methods whenever possible, such as cell cultures or computer models. Reduction means designing experiments to use the fewest animals that can still produce reliable results, including using statistical power analysis to calculate exact sample sizes rather than defaulting to large groups. Refinement means minimizing suffering during experiments.
In practice, the 3Rs have led to real changes. Computer modeling and cell-based testing have replaced some animal studies entirely, and better experimental design has reduced the number of animals needed per study. But these gains have not fundamentally changed the endpoint for most laboratory animals. When tissue examination is the goal of the study, or when animals carry diseases or genetic modifications that make rehoming impractical, euthanasia remains the standard outcome. The 3Rs reduce the total number of animals that reach that endpoint, but they do not eliminate it.