Lab animals are also used to establish lethal doses, a basic measure that determines concentrations at which a toxic chemical is lethal to 50 percent of a test population.

Still, most rats up for adoption at the E.P.A. are sentinel rats, which are used to test for contaminants in the environment. “They just hang out,” said Paula Clifford, executive director of Americans for Medical Progress, a nonprofit that advocates the responsible use of animals in research. “Researchers might check their fecal matter,” she said, or “they might take some blood, and then they test that to see, are there any contaminants in the environment?”

Shutting that research down without developing alternatives could not only risk public health, she said, but could also make E.P.A. scientists more dependent on industry research and less able to verify the safety of chemicals.

Despite E.P.A. staff members’ efforts, some lab animals may not be suitable for adoption. Ones that have been genetically modified or have certain diseases aren’t up for grabs, a person with knowledge of the adoption program said.

Ms. Hirsch, the E.P.A. press secretary, said that animals “ineligible for adoption due to their exposure to harmful pollutants will be given humane end-of-life care.”

As for eligible rats, they make great companions, according to Ms. Clifford, who said she had adopted many former lab residents. “If you can convince people to have rodents in their homes, they’re wonderful pets.”