Hazel, a 6-year-old beagle, got scared her first night in a foster home when she played with a toy and it squeaked. She had never seen a stuffed animal or ball before. She was comfortable with water but afraid of the tub during her first bath.

Hazel is one of more than 400 beagles who were released from a breeding facility in Virginia last week. About 4,000 total are expected to be released to shelters, rescues, foster owners and adoptive families in the next two months.

The mass rescue comes after United States authorities filed a complaint in a federal court in May, after inspections of the Envigo breeding and research facility in Cumberland, Va., over the past two years revealed several violations of federal regulations. Officials found the beagles hungry, sick, mistreated and, in some cases, dead. Many of the animals in the breeding operation were expected to be used in research and testing. After the inspections and calls from lawmakers, a federal judge approved a plan this month to rescue the beagles. That mobilized several rescue organizations, dozens of volunteers and hundreds of would-be owners who wanted to help.

Hazel took her first walk on Tuesday in the care of Nikki Bunce, who is a first-time foster owner for the dog and her five puppies in West Bend, Wis. She said Hazel had warmed up to cuddling during movie nights.

“It’s just been so heartwarming to be able to be her first everything,” Mrs. Bunce said.

Working to rescue, medically treat and relocate the dogs has been an enormous undertaking that has required the help of veterinarians, volunteers, drivers and dog lovers.

Envigo, a research organization that was acquired last year by Inotiv and works with the pharmaceutical and biotech industries, said on its website that it breeds “healthy, well-socialized animals.”

On July 21, the Humane Society of the United States took 201 beagles, among the first to leave Envigo, to a center in Maryland, and about 230 other dogs went directly to rescue partners. Workers in personal protective equipment carried the dogs off vans and inspected them before taking them into the rehabilitation center.

The dogs were previously identified using tattoos on the inside of their ears, which is how the breeding operation had tagged them. One puppy had the letters “ONE CJE” on the inside of its left ear. Their foster and adoptive families are now naming them beyond those codes for the first time.

Before the court intervened, some of the dogs had likely been destined to end up at testing facilities and die, said Kitty Block, the chief executive and president of the Humane Society.

“They deserve to be on couches, on dog walks with you in the park,” Ms. Block said.

Lindsay Hamrick, the Humane Society’s shelter outreach and engagement director, said pregnant dogs, nursing litters and dogs in need of medical care were prioritized for new homes. Those that have been rescued will undergo additional veterinary examinations and have paperwork prepared so they can be adopted across the country. The Humane Society said it planned to help rescue about 300 to 500 beagles weekly until they were all settled.

After a few weeks of a normal, healthy routine, most dogs adapt well to new homes, Ms. Hamrick said. But in some cases, dogs might need years to adjust to “normal life,” she said.

“Everything, from the way that grass feels to watching cars drive by, it’s all going to be a brand-new experience for them,” Ms. Hamrick said.

Of a group of 62 beagles in Wisconsin, the nine mothers, who grew into adulthood at the Envigo facility with little human interaction or play, have been shy, Angela Speed, the vice president of communications for the state’s Humane Society, said.

Two drivers transported the beagles in large cargo vans — nine moms and their 53 puppies — from Maryland to Wisconsin, where 15 staff members and volunteers in Milwaukee received them and prepared them to go to foster homes that night.

“Their lives have been completely transformed,” Mrs. Speed said. “Animal lovers step up to help, and that’s what makes this possible.”

A separate effort in Massachusetts required two large vehicles, more than 20 hours on the road and three drivers who took 75 beagles to the Northeast Animal Shelter in Massachusetts, Mike Keiley, the organization’s executive director, said. Of those, 20 went to the Dakin Humane Society in Springfield, Mass., and the remaining 55 are in the care of the shelter.

“We jumped on the opportunity to help with such a historic and meaningful case that, in my opinion, really points an important spotlight in a dark corner of animal welfare,” Mr. Keiley said.

Aside from natural disasters that have displaced some dogs, Mr. Keiley, who is also the adoption coordinator for the Massachusetts Society for the Prevention of Cruelty to Animals, said the rescue of the 4,000 beagles was the largest he had participated in or heard of. The shelter said 800 people reached out to inquire about adopting a member of this batch of puppies, all eight to 12 weeks old, or any additional ones they house in the future.

“You would expect them to be fearful of people, not trusting, and really traumatized,” Mr. Keiley said of the puppies. But that hasn’t been the case. “I’m really impressed with how resilient animals are coming out of some of the worst situations you could imagine,” he said.

The beagles have to undergo medical care and vaccinations that are specific to each state. In Massachusetts, this includes a quarantine period, P.P.E. for caretakers, vaccinations, microchipping, parasite treatment, and spaying or neutering, said Karina King, director of operations at the Dakin Humane Society.

So far, many of the society’s 20 beagles have diarrhea and one will have an eye surgically removed, Ms. King said. The shelter will take care of many medical needs before the dogs are adopted, and any with persisting issues will go to foster homes until they can recover.

Ms. King said interest was high in the beagles during a time when animal shelters across the country were strained. Her shelter has fielded requests from Texans and Floridians willing to drive to Massachusetts for a beagle, even though there are dogs ready to be adopted in those states.

“If these dogs’ story catches your heart, that’s great if you’re able to get one,” Ms. King said. “But, if you’re not, there are so many other dogs who need your help.”

Nellie (named because she is a “nervous Nellie”) was originally fostered through Homeward Trails Animal Rescue in Virginia and adopted within 24 hours by Lauren and Trevor Kellogg in Washington, D.C. Both had done advocacy work against animal testing and Envigo, Mrs. Kellogg said.

Nellie, 2 years and 8 months old, joined the almost 4-year-old Beesly, another beagle rescued from animal testing, who was also timid and wary when adopted two and a half years ago. Mrs. Kellogg said she used to work for a pharmaceutical company and adopted Beesly after the dog was part of an experiment her company conducted.

After their release, the 21 dogs who were put in the care of Homeward Trails Animal Rescue in Virginia had a “spa day,” said Sue Bell, the executive director. For the first time, they could run around a sunny yard to dry off after a bath.

“Previously, when we’ve taken dogs, I have looked in the eyes of the beagles in their outdoor kennels row after row after row and kind of had to say an apology to them,” Ms. Bell said. “This time, I was able to look in the eyes of all those dogs and tell them we were coming back for them.”