Nelson Mandela was admitted to a hospital in Pretoria on Saturday to undergo tests and was said to be doing well, the South Africa government said.
The 94-year-old former president and anti-apartheid icon was admitted for tests "consistent with his age", the office of President Jacob Zuma said.
The government insisted there was "no cause for alarm".
Zuma spokesman Mac Maharaj, once a fellow prisoner with Mandela, told AFP that while the tests were "not routine", they were "normal tests that a person of his age undergoes".
Officials refused to give more details about his condition or say which hospital he was at, instead appealing that his privacy be respected.
The revered statesman has not appeared in public since South Africa's Football World Cup final in 2010.
Madiba, as he is affectionately known by South Africans, has all but retired from public life, choosing to live in his childhood hometown of Qunu in the rural Eastern Cape.
Villagers in the town reported a slightly unusual movement of police around Mandela's Qunu homestead earlier in the day.
He was later flown from Qunu to the capital Pretoria, around 900 kilometres (560 miles) away.
A traditional ruler for the village, Nokwanele Balizulu, told AFP she saw Mandela shortly before he was taken to hospital.
"I was called by the Mandela family saying Tata (grandfather) is not well. I rushed there and I saw he is not well," she said in the local Xhosa dialect.
Rumours of his failing health or even death flare up periodically, forcing the government to issue assurances to calm the speculation.
His last hospitalisation was in February when he spent a night in hospital for a minor exploratory procedure to investigate persistent abdominal pain. Those tests showed there was nothing wrong with him.
In January 2011, Mandela was admitted for a chest infection, sparking public panic and a media frenzy as the government and Mandela's charitable foundation refused to release information on his condition.
In May, a smiling Mandela made a television appearance to receive a flame to mark the ANC's centenary at his home, but the once spry boxer, who stayed fit during his 27 years in prison by doing calisthenics in his cell, has grown increasingly frail.
His stature as one of the world's most famous and loved public figures remains undimmed.
After years fighting white-only rule, he shared the Nobel Peace Prize with the last white president, FW de Klerk, in 1993.
A year later, he crowned his long fight against minority rule by becoming the country's first black president with the end of apartheid.
After ushering in a period of immense change he handed over to Thabo Mbeki in 1999.
Lately he has received visits from US Secretary of State Hillary Clinton and her husband, former US president Bill.
Both reported that the he was doing well and in good spirits.
He has also been visited by Zuma, who on Saturday said Mandela's medical team would get all "our support as they look after and ensure the comfort of our beloved founding President of a free and democratic South Africa".
The leading labour union confederation Cosatu expressed its "heartfelt best wishes to our icon and national leader."