Five hundred young water voles are to be released into the River Dore in Herefordshire with the aim of boosting numbers of the threatened creatures.
The animals have been raised in captivity and experts hope they will breed in their new home.
In order to give the tiny creatures the best chance of survival, all American mink have been removed from the river.
Mink are their main predators and are thought to be the biggest reason for the fall in the UK vole population.
Water vole facts
Live in burrows on river banks
Are hunted by mink, stoats, owls, herons, foxes and cats
Their homes are threatened by farming methods
Their greatest threat is the American mink
Numbers have dropped dramatically over the last 100 years and loss of natural habitat is also thought to be partly to blame.
The first group of young voles will be released on Friday. It is one of 50 colonies to be freed along 18 miles of the river bank.
Dr Jonathan Reynolds, of The Game Conservancy Trust, said: "Water voles were once very common on the River Dore but like many other places in the UK they have completely disappeared in the past 20 years."