Mandela's office said it was just for routine tests, but it has still drawn concern of the health of the anti-apartheid icon.
Mandela undergoes regular hospital checkups, but his latest visit has stretched into an unusually long stay. Journalists camped outside the hospital Thursday as Mandela's relatives and friends entered for visits.
President Jacob Zuma, attending the World Economic Forum annual meeting in Davos, Switzerland, is being updated on developments by the defense minister, whose department is responsible for current and former presidents' health care, Zuma's office said.
Zuma's office said Mandela "is comfortable and is well looked after by a good team of medical specialists."
Mandela was jailed for 27 years for his fight against apartheid. He became South Africa's first black president in 1994 and stepped down after serving one term in 1999. He largely retired from public life in 2004.
The Associated Press contributed to this story.