Northern White rhinos are sub-species of the White rhino
Four rare Northern White rhinos have been flown from a Czech zoo to Kenya, in a desperate attempt to save the species from extinction.
Animal experts hope the rhinos - two males and two females - will breed in their natural habitat in Africa.
Only eight Northern White rhinos are known to survive worldwide, all of them in captivity: six in the Czech Republic and two in the US.
The last four living in the wild in Africa have not been seen since 2006.
On Sunday, the four rhinos from the Dvur Kralove Zoo in the Czech Republic were packed into crates and loaded onto a Boeing 747 bound for the Ol Pejeta reserve in central Kenya as part of the "Last Chance to Survive" project.
"We plan to give the remaining individuals with breeding potential their last chance of normal and regular reproduction in a secure location in the wild," said zoo director Dana Holeckova.
Rob Brett, member of the African Rhino Specialist group at the International Union for Conservation of Nature (IUCN), said: "Moving them now is a last-bid effort to save them and their gene pool from total extinction."
"They are listed as critically endangered on the IUCN Red List of Threatened Species and are thought to be extinct in the wild," he added.
Attempts at Dvur Kralove to breed the animals have produced just one offspring in 10 years: a female rhino was born in 2000 and later dubbed the zoo's "millennium child".
It is now hoped that returning the rhinos to their natural habitat in Kenya will make the females more fertile and the males more interested.
However, the European Association of Zoos (EAZA) has expressed concerns that years in captivity makes the undertaking dangerous and unpredictable, the BBC's Rob Cameron in Prague says.
Defenders of the plan reject that, saying they believe the animals' genetic memory will quickly take over, our correspondent adds.