Twenty pairs of captive-bred dormice have been released in a secret woodland location in Nottinghamshire to help boost populations.
The rodents, which can hibernate for up to six months, were once widespread but the species is now vulnerable to extinction.
Conservationist Ian White said the animals have suffered due to habitat loss and poor management of woodland.
He said survival in the wild for captive dormice is "extremely high".
Health checks have been conducted by vets at Paignton Zoo, in Devon and the Zoological Society of London.
Each breeding pair has been provided with a secure wooden nest box fitted inside a mesh cage, and secured to trees.
The People's Trust for Endangered Species (PTES) said this will help the dormice adjust to their new home in the wild.
The rodents will be fed daily for a few weeks and then will be allowed to explore their new habitat.
Ian White, from PTES, said: "Survival rates of dormice reintroduced into the wild are extremely high.
"Over the last 50 years, they have been lost from all the midland and northern counties probably due to habitat fragmentation, and poor management of woodlands."
Mr White added dormice from five release sites in England are known to have successfully dispersed beyond the woodland in which they were released.
Nottinghamshire Wildlife Trust will manage the woodland to ensure the habitat has suitable food and shelter for the animals.
PTES said it has released 720 dormice across 11 counties over the last 20 years.