Kenya has begun moving 400 elephants from one national park to a larger one in what it calls the biggest transport of animals "since Noah's Ark".
The first elephant to be tranquilised was a 22-year-old bull
The animals, which weigh two to four tons each, are being shot with tranquiliser darts, loaded onto special trucks and driven eight hours north.
There was a serious overcrowding problem at the Shimba Hills Reserve, the Kenya Wildlife Service (KWS) said.
The operation will cost $3.2m (£1.8m) and will take about eight months.
Shimba Hills has a population of about 600 elephants - but capacity for only 200.
The animals are being transported to Tsavo East National Park, which is more than 70 times larger than Shimba Hills.
"The relocation will save Shimba Hills... from impending ruin," KWS scientist Patrick Omondi said, according the AFP news agency.
Tsavo East once had a population of more than 25,000 pachyderms, but poaching in the 1980s and 1990s reduced the herd to the present 10,397.
KWS director Julius Kipngetich said 83 additional rangers were being stationed at the park to prevent poaching.
400 elephants being moved
Animals being driven 350km (217m) - an eight-hour journey
Operation to cost $3.2m (1.8m)
"If the poachers come, they will find us ready," Mr Kipngetich said.
Six of the animals are also being fitted with tracking devices so game wardens can keep them away from farmland.
"We will be monitoring their movements using GPS so that our rangers can drive them away before they reach private farms," Mr Kipngetich said.
The first animal to be tranquilised was a 22-year-old male.
But plans to move him on Thursday were scuppered by problems with the weather and with the truck.
Bad weather grounded a helicopter surveillance flight that was to have been part of the operation, KWS spokesman Edward Indakwa said.
The KWS had planned to start moving the animals in family groups on Saturday, but the problems moving the bull put the schedule into doubt, the Associated Press reported.
Elephants live in female-led groups, while adult males live on their own.