Mac Maharaj, spokesperson of the South African government, said the admission of 94-year-old Mandela was a scheduled affair for someone of his age.
Official statements said the doctors were conducting the tests and there was no reason to worry.
The iconic leader was admitted in a Pretoria hospital although authorities did not specify its name.
Maharaj did not specify how long Mandela would be kept in the hospital.
Mandela, who served as his country's first black President between 1994-99, is widely revered as the father of the nation for leading the long struggle against apartheid and for democracy.
The leader had been admitted to hospital thrice in the past two years. In January 2011, he was treated for a chest infection in Johannesburg. He was admitted to hospital for 18 days in December last year.
The former president has rarely made a public appearance since retiring from public life in 2004.