Animal testing in research facilities involves housing animals in controlled laboratory environments and exposing them to drugs, chemicals, or procedures to evaluate safety and effectiveness before human use. The experience varies enormously depending on the species, the type of study, and the facility. An estimated 17 to 22 million animals are used annually in the United States alone, with roughly 85 percent being rats and mice. Less than 2 percent are cats, dogs, or primates.
What Happens During a Typical Study
Most animal testing falls into one of two broad categories: safety testing and disease research. In safety testing, animals receive controlled doses of a substance, whether a pharmaceutical compound, cosmetic ingredient, or industrial chemical, to see how their bodies respond. Researchers start with short-term studies, administering single or multiple doses and observing the animals over set periods. The highest dose given is usually based on the maximum the animal can tolerate without dying, not the dose that would eventually be used in humans.
For drugs intended for daily human use, the testing timeline mirrors the planned clinical trial. If a human trial will last 14 days, the animal study must run at least 14 to 28 days. Later-stage clinical trials require longer animal studies, often 60 days or more. Throughout these periods, researchers monitor weight, behavior, organ function, and blood work. At the end of most studies, animals are euthanized so their tissues and organs can be examined for damage.
Disease research looks different. Animals may be genetically bred to develop conditions like cancer, diabetes, or Alzheimer’s disease, or they may undergo surgical procedures to model injuries. Researchers then test potential treatments and track whether the animals improve, stay the same, or decline.
How Pain Is Classified and Managed
The USDA requires research facilities to categorize every animal by the level of pain or distress involved in its use. Category C covers procedures that cause no pain at all, like behavioral observation or blood draws under brief sedation. Category D includes procedures that do cause pain but where the animal receives anesthesia, painkillers, or sedatives. Category E is the most controversial: procedures involving pain or distress where pain relief cannot be given because it would compromise the results of the study.
Category E experiments might include certain toxicity studies where researchers need to observe the full, unmedicated progression of a substance’s effects, or pain research itself. These studies require extra justification and review before they’re approved. The majority of animal research falls into Categories C and D, meaning most animals either experience no pain or receive medication to manage it.
Living Conditions Inside the Lab
Laboratory animals live in enclosures regulated by federal standards that specify minimum floor space, vertical height, and temperature ranges for each species. Dogs, cats, and primates have different space requirements than rodents, and nursing mothers with young require additional room. Every facility must have a veterinarian on staff who provides routine and emergency care, including nights, weekends, and holidays.
Federal guidelines also require environmental enrichment, meaning the animals’ living spaces should include more than just food and water. Social species like primates and rats are supposed to be housed in pairs or compatible groups rather than alone, unless the experiment specifically requires isolation. Enclosures for group-housed animals include features like perches, visual barriers, and hiding spots so that dominant animals can’t monopolize food and space. Rodents receive nesting materials for warmth and comfort. The gap between what regulations require and what individual facilities actually provide, however, is a persistent point of criticism from animal welfare organizations.
Who Oversees the Process
Every research institution that uses animals must maintain an Institutional Animal Care and Use Committee, or IACUC. This committee reviews and approves every proposed experiment before it begins. At least one member must be a licensed veterinarian with laboratory animal experience, and the committee evaluates housing, care, and treatment standards across the facility. Researchers must justify why they need animals rather than an alternative method, explain what species and how many they need, and describe what pain management they’ll use.
The Animal Welfare Act sets the legal floor for these standards, but it has a significant gap: rats, mice, and birds bred for research are excluded from its protections. Since those species make up the vast majority of lab animals, most research animals fall outside the Act’s direct oversight. Institutions using these species still typically follow guidelines from the National Academies, but compliance is largely voluntary and tied to funding requirements rather than federal law.
What Happens to Animals Afterward
Most laboratory animals are euthanized at the end of a study. For rodents, the most common methods include injectable sedatives that cause rapid unconsciousness followed by cardiac arrest, or carbon dioxide exposure under specific conditions. Physical methods like cervical dislocation are permitted for small mice and rats under certain circumstances. The American Veterinary Medical Association sets detailed standards for each method, with the core requirement being rapid loss of consciousness before death.
Adoption or retirement is possible for some larger animals, particularly dogs and primates, and several states have passed laws encouraging or requiring facilities to offer healthy animals for adoption when studies end. In practice, adoption remains uncommon relative to the total number of animals used, partly because many studies require tissue analysis that can only happen post-mortem.
The 3Rs Framework
Modern animal research operates under a principle called the 3Rs: Replacement, Reduction, and Refinement. Replacement means using non-animal methods when they exist. Reduction means designing experiments to use the fewest animals possible while still producing valid results. Refinement means minimizing suffering through better anesthesia, less invasive techniques, or choosing a less severe experimental approach when options exist.
In practice, Reduction often comes down to better experimental design and statistical planning. Using well-controlled conditions and genetically uniform animal populations reduces variability, which means fewer animals are needed per experiment to detect a real effect. Refinement shows up in decisions like using imaging technology to track disease progression in a living animal rather than euthanizing groups of animals at different time points to examine their tissues.
Alternatives Gaining Ground
One of the most significant developments in reducing animal testing is organ-on-a-chip technology. These are small devices, roughly the size of a USB drive, that contain living human cells arranged to mimic the structure and function of real organs. Since 2010, researchers have built chip-based models of nearly every major organ system: brain, lung, heart, liver, kidney, gut, skin, bone marrow, and others. Each can be used to test how a drug is absorbed, whether it causes toxicity, or how it interacts with disease.
More advanced versions link multiple organ chips together through shared fluid channels, simulating how a drug moves through the body. A liver-heart chip, for example, can reveal whether a compound that’s safe for liver cells might be toxic to heart tissue. Liver-lung-cancer chip combinations have been used to study how tumors spread between organs and how inhaled versus injected therapies compare. These systems can even model cross-species differences, testing how a drug behaves in human, rat, and dog cells side by side on the same platform.
Organ-on-a-chip models don’t yet replace animal testing entirely. Regulators still require whole-animal studies for most drug approvals because chips can’t capture the full complexity of a living immune system, hormonal feedback, or behavior. But they’re increasingly used in early drug screening to filter out toxic compounds before animal studies begin, which reduces the total number of animals needed downstream.